Notes on the Materials of Research

Source: Procedures in Experimental Physics
by John Strong

Alkali metals. One of the alkali metals may be required for the sensitive surface of a photocell or as a thin-film filter for ultraviolet light; or in the vapor phase the metal may be used for the demonstration of the phenomenon of resonance radiation. For these and other applications we will outline, briefly, some of the ways of manipulating these very reactive metals.

The alkali metal may be prepared from the alkali chloride, reduced with calcium metal in an evacuated glass tube:

2MCl + Ca + CaCl₂ (1)

The reaction progresses in the indicated direction at elevated temperatures on account of the removal of the free alkali metal, M, by evaporation. This reaction may be varied: A chromate of the alkali metal may be used instead of the chloride, and zirconium metal may be used instead of calcium. The reaction applies to the preparation of all the alkali metals, with the exception of lithium, which reacts with the glass or quartz; lithium is best reduced from its chromate with zirconium metal in an iron apparatus.

We will consider, in detail, how potassium may be prepared by the reaction indicated in Eq. 1. Pulverized potassium chloride and calcium metal filings are mixed together in a closed-end iron tube in stoichiometrical proportions (3.7 g KC1 to 1 g Ca). This iron tube is introduced into the thickened end of a hard-glass tube as shown in Fig. 1. The glass is thickened in order to allow the attainment of the maximum temperature; at lower temperatures where a thinner glass wall would collapse the reaction proceeds very slowly. After the iron tube is introduced, the hard-glass tube is closed by fusing the glass with the hand torch. After a good vacuum is attained, the chemicals are heated, slowly at first and finally strongly until the reaction is complete. The chemicals may be heated until the glass starts to soften, but too much heat should be avoided, since it will distill calcium metal. The reduced and once-distilled metal condenses in the bend of the tube as shown in Fig. 1. From there, it is worked down with the flame into the receiving ampoule, where it is sealed off as illustrated.

The alkali metals react vigorously with air, and the ampoule should be opened without exposing the metal to air. This is done by the following procedures: The ampoule is constructed of Pyrex glass with an annular tungsten ring to spring the glass. After the ampoule is mounted in the vacuum system, the tungsten ring is heated with a high frequency induction coil until the glass breaks. (See Fig. 2.)

A scheme which does not involve the use of high-frequency heating but which breaks the glass by impact is illustrated by Fig. 3. The illustrated depression in the tube wall acts as a safety to confine the armature and prevent accidental fracture of the ampoule until the apparatus is sealed onto the high-vacuum system. During this sealing operation the depression is blown out of the way of the armature. The armature is operated in the vacuum, by means of an external electromagnet, to break the tip of the ampoule, thus exposing the alkali metal. The tip may be scratched with a file to facilitate breaking.

The ampoule may be cooled in a beaker with dry ice in the bottom and carbon dioxide vapor above. The tip is opened under the surface of the carbon dioxide vapor. The ampoule is then quickly transmitted to the vacuum system, sealed in, and evacuated. The expansion of carbon dioxide in the ampoule through the tip prevents access of air to the alkali metal.

The alkali metals, as obtained commercially in small cubes or irregularly shaped pieces, are packed submerged under kerosene. The metal may be cleaned and manipulated as follows:¹ First the metal is washed in dried petroleum ether or benzene to free it from kerosene. The petroleum ether or benzene is dried by shaking it in contact with calcium chloride. (Carbon tetrachloride or chloroform should not be used to wash the metal, since an explosive compound is formed.) The metal is then fused in the bottom of an 1-mm glass tube and sucked up into a 1-mm capillary glass tube with a rubber hose. This 1-mm tube is sealed off with a flame just above the metal. At the other end the alkali is protected from the air by soft wax. A suitable length of this composite glass-metal rod may be cut off with wire cutters and introduced into a distilling bulb fastened to the vacuum system where the metal is desired. (See Fig. 4.)

A distillation procedure² for sodium metal is illustrated in Fig. 5 whereby the metal is refluxed under vacuum to free it of hydrogen and carbohydrates (the hydrogen contained in potassium or sodium, measured as a gas at atmospheric pressure, may amount to one or two hundred times the volume of the metal). The metal cubes are washed to free them of kerosene, as described above, and then they are introduced into Chamber I. After the whole system is evacuated, the metal is fused in this chamber. Chamber I acts as a separating funnel. The fusion is accomplished by the application of a soft flame, so that the metal runs into Chamber II, leaving the dross behind. Chamber I is then removed at the seal-off. The metal is heated in Chamber II with small electric furnace. Here it is refluxed for several hours. The distilled metal condenses in the asbestos-insulated tube above Chamber II. This refluxing allows hydrogen and hydrocarbon vapors to be pumped away. After this treatment the metal is distilled into Chamber III by a heater wire around the condenser tube. Chamber II is then removed at the seal-off. Chamber III may be the receiver for the metal, or it may be further refluxed and distilled into a final receiving ampoule. Electric heat is recommended for distilling the alkali metal, since there is some danger of breaking the glass if it is heated with a torch.

In manipulating the alkali metals the following precautions should be observed: The amount of metal manipulated should never be greater than necessary. A box of sand should be at hand for the control of accidental fires. The alkali metals should never be allowed to come in contact with water. Used metal and apparatus containing the alkali metals should be disposed of by burying only. It is advisable to wear goggles to protect the eyes while manipulating the alkali metals.

Sodium may be prepared by electrolysis through the soda-glass walls of an electric lamp. A 32-volt lamp, which has a larger tungsten filament wire than the 110-volt lamp, is best for this purpose. The lamp bulb is first evacuated by means of a side tube sealed on for this purpose. It is then dipped in a bath of fused sodium nitrate and nitrite and connected to a source of electrical energy as shown in Fig. 6. Current is carried from the tungsten filament to the glass walls of the lamp bulb by electrons or by means of a sodium discharge, or in special cases by means of an argon discharge. The practical details of this procedure are due to Dr. R. C. Burt, who graphically described the procedure as one which allows the vacuum to be electroplated with sodium.³ The free metal is formed from the reduced sodium ions (which migrate through the solid glass electrolyte when a current flows). These ions are reduced by electrons, or negative sodium (or argon) ions. Faraday's law applies to the electrolysis. The spectrum of the sodium vapor discharge has been photographed and the spectrum indicates high purity of the electrolyzed metal. Impurities were estimated by Dr. Burt as being present, at most, in the proportion of 2 parts per million. Sodium prepared by electrolysis is characterized by the fact that it is completely free of hydrogen and carbohydrates.

The electrolysis current varies from a few milliamperes when the current is carried entirely by electrons to a few hundred milliamperes when it is carried by sodium ions. The sodium discharge is obtained by simply removing the air blast on the lamp bulb which normally serves to keep the metal condensed.

Burt states that the spectrum from the sodium discharge is not reversed, a warmed lamp containing sodium will fluoresce if the light of the sodium discharge from another lamp is focused on it. (See Fig. 7.)

Sodium may be introduced into quartz photocells by means of a graded seal as shown in Fig. 8.

Potassium can be electrolyzed through a potassium glass which is free of sodium and lead. A bath of fused potassium nitrite and nitrate is used.

The alkali metals potassium and sodium may be dissolved in the volatile solvent, liquid ammonia, and deposited where they are desired by boiling away this solvent. Lithium is managed in a similar manner with aethylamine as solvent.

All the alkali metals react with glass at elevated temperatures and especially with lead glass, with which they should not be allowed by come in contact.

The resistance of Pyrex-glass tubes toward sodium can be improved if they are lined with a film of borax or boracic acid. The tube to be lined is filled with a hot saturated solution of borax. The borax precipitates from this solution as crystals on the inner glass walls of the tube as the solution cools. When the glass has become lined with a thin coating of crystals, the solution is drawn off and the tube carefully dried. It is then evacuated, and the water is driven off by heating. At first the heating is gentle, but finally the tube is fired at the maximum temperature the glass will stand. This gives the tube a smooth sodium resistant inner surface.

The potassium-sodium alloys, lying within the composition range 45 to 90 per cent potassium, are liquid at room temperature.

Alkali-earth metals. The chief uses of the alkali-earth metals, as getters, depend on their reactions with oxygen to form oxides, with carbon dioxide to form carbides and oxides, with water to form hydrides and oxides, and with nitrogen to form nitrides.

When fresh calcium filings are heated in a quartz tube or heavy-walled Pyrex side tube, connected to an apparatus such as a thermopile, the calcium reacts with all the residual gases (except the noble gases). A fairly good vacuum can be obtained with such a side tube even when starting at atmospheric pressure. For example, the argon spectrum may be obtained in a discharge tube evacuated from half of an atmosphere pressure with such a calcium side tube. Each time the tube is evacuated from atmospheric pressure with calcium, the residual pressure of argon (calculated from its abundance in the atmosphere) is increased by 7 mm.

Barium is a more reactive metal than calcium.⁴ It is used as a getter for commercial radio tubes. For this application, the metal is sometimes cast in a seamless tube of nickel or copper which is drawn down to wire. These composite wires are known as Niba and Cuba wires. The wires are cut into short lengths, which are introduced into radio tubes and other places where the getter action is desired. The volatile core metal is subsequently boiled out of the nickel or copper covering tube by means of heat generated with a high-frequency induction coil.

Mercury. Although mercury approaches the noble metals in chemical inertness, it is easily contaminated, especially by other metals. This is because, as a liquid, it is a fairly good solvent. A simple test for the purity of a sample of mercury is to raise a clean glass rod slowly up through the metal surface. If the mercury is clean, the glass will come up without any adhering mercury droplets.

The contaminations commonly found in mercury may be classified according to the manner in which they can be easily removed. First come surface contaminations by materials which do not dissolve in the liquid metal and may, accordingly, be removed by filtering the metal through pinholes in filter paper or through a chamois skin. Second, there are the dissolved metals. Those which are oxidizable are first converted to insoluble oxides by the blowing of air through the mercury as shown in Fig. 9(a). The oxides form a scum on the mercury surface and may later be filtered off. Mercury is practically free of impurities of this type if, after air has been blown through the liquid metal 2 hour, no scum has formed on the surface. The alkali metals fall into this class of impurities; here also belong zinc, with a high vapor pressure, and copper and lead, with low vapor pressures. These metals, which are more reactive than mercury, can also be removed by exposing the mercury to a solution of 10 per cent HNO₃ or 80 per cent H₂SO₄. This is shown in Fig. 9(b). Thirdly, there are the dissolved metals, such as the noble metals and tin, which cannot be removed by oxidation or acid. Copper and lead may also be considered as belonging to this class of contaminations. These metals are removed by vacuum distillation of the mercury at a temperature of about 180 to 200C. (at which temperature the mercury distills at the rate of approximately 1/2 g/cm²/sec.) as indicated by Fig. 9(c).

Platinum metals. Platinum is chemically resistant to alkalies and hydrofluoric acid. However, it is attacked by chlorine vapor and aqua regia. Metallic salts should not be heated in platinum under conditions which may result in the reduction of the metal and the consequent debasement and embrittlement of the platinum. This applies particularly to lead salts. The elements phosphorus and silicon also attack platinum and make it brittle, and they may change its other properties. For example, even the small amount of silicon introduced into the platinum when it is heated in contact with porcelain in a reducing atmosphere makes an appreciable change in the thermoelectric power and electrical resistance.

Platinum is so ductile that wires may be drawn directly as fine as 20 diameter. By Wollaston's procedure a platinum rod is covered with a close-fitting silver tube, and this composite rod is drawn through wire dies. After the silver has been etched off the final wire with nitric acid, the platinum wire obtained may be as small as 1/2 in diameter. Wollaston wire is often used for fuses to protect delicate instruments.⁷

A physical property of platinum which is of interest to the physicist is its "transparency" to hydrogen gas at temperatures above 700C. (See Fig. 10.) This property is employed to obtain very pure hydrogen.

Platinum is a refractory metal. For this reason it may be used for furnace windings and as a base for oxide cathodes.

Iridium is harder and more resistant to chemical attack than platinum; it is not attacked by aqua regia. Accordingly, it is often alloyed with platinum in proportions up to 30 per cent to yield a metal which is superior to platinum in respect to chemical resistance and hardness.

Rhodium is alloyed with platinum (90 Pt to 10 Rh) to yield the LeChatelier thermocouple alloy. Rhodium is a bright inert metal and for this reason it is used for electroplating other metals.

Osmium is the most refractory metal of the platinum family, with a melting temperature of 2700C. It was once used in incandescent lamps but has now been replaced by tungsten for this use. Incidentally, it is the heaviest known substance, having a density of 22.5 g/cm³.

Palladium is the least noble of the platinum metals. It oxidizes when heated in air and is dissolved in nitric acid. 3 Hydrogen diffuses through palladium more rapidly than through platinum. At atmospheric pressure palladium will ~ dissolve about 6 mg H₂ per 100 g of metal to form the " alloy " Pd₂H. The hydrogen is given off again if the metal is heated, in vacuum, to temperatures above 300C. (See Fig. 10.) This property affords a convenient source of extremely pure hydrogen in small quantities.

The refractory metals: Tungsten, molybdenum, tantalum, and so forth. Tungsten is the most refractory metal and also the strongest. Wires of .0014 inch in diameter exhibit a tensile strength of 590,000 lbs./square inch. Tungsten is quite "unorthodox" in its behavior with respect to cold working and heat. Passing it through dies makes it more ductile, while heating it to a temperature greater than 1000C. causes recrystallization and makes it brittle, a situation just opposite to the behavior of most metals. The ductility of tungsten at ordinary temperatures is due to its long fibrous crystal grains. Fig. 11 shows the--ductility of tungsten at various temperatures. It will be noted that recrystallized brittle tungsten is ductile if heated to temperatures greater than 200C.

Traces of water vapor are corrosive on the tungsten filaments in vacuum electric lamps. The water molecule reacts with hot tungsten to form tungsten oxide and atomic hydrogen, both of which evaporate to the glass wall of the bulb, where, owing to catalytic effect of the glass, they react to give metallic tungsten and water vapor again. The water molecule is now free again to repeat its action on the tungsten filament.

Tungsten reacts with oxygen and carbon monoxide, in vacuum, to form oxides and carbides. Tungsten is not attacked or affected by mercury vapor or hydrogen gas. In air, at a yellow heat, tungsten reacts with oxygen to form volatile oxides, which distill off as white smoke.

Molybdenum is more ductile than tungsten. Otherwise, it is very similar to tungsten, and the two metals form alloys in all proportions. Some of these alloys are used commercially. Their properties are, in general, a compromise between the higher melting temperature of tungsten, on the one hand, and the greater workability and machinability of molybdenum, on the other.

Molybdenum and tungsten do not soft-solder or amalgamate with mercury, but both metals may be welded to nickel or Advance alloy. Nickel is frequently welded to tungsten to facilitate connecting it by spot-welding, soldering, or brazing to other less refractory metals.

Tungsten or molybdenum may be cleaned by heating the metal to a red heat and rubbing its surface with a piece of potassium or sodium nitrite.

In many respects tantalum is like molybdenum and tungsten.⁸ Tantalum, when it is very pure, is one of the most ductile metals. However, when heated in hydrogen or air, tantalum becomes brittle. To anneal tantalum, it must be heated to about 800C. in a vacuum better than mm of mercury. Because tantalum readily gives off occluded gas if heated above 800C., it is used as a construction material in vacuum tubes.

To spot-weld this metal successfully, it must be submerged under carbon tetrachloride or water. It may be machined using carbon tetrachloride as a cutting fluid, and spun using hard laundry soap as lubricant.

Columbium occurs with tantalum and has many properties in common with it. It is less refractory and more ductile than tantalum. It is used as a substitute for tantalum.

Rhenium is the heaviest member of the manganese subgroup in the periodic table, and it is very refractory, its melting temperature being only about 200 below that of tungsten.

Alloys. Invar. The iron-nickel alloy, 63.5 Fe, 36 Ni, 0.5 Mn, is known as Invar. Its coefficient of expansion is only low for temperatures below 120C., being per degree centigrade. The heat conduction of Invar is also very low, being only 1/40 that of copper. Invar does not corrode. It is used for the construction of surveyor's tapes and instruments in which the dimensions are required to remain constant in spite of temperature changes. The alloy melts at 1425C.

Electrical-Resistance alloys.⁹ Nickel-chromium alloys are characterized by a high electrical resistance (about 58 times that of copper), a low temperature coefficient of resistance, and a high resistance to oxidation. Examples are Chromel A and Nichrome V, of which the typical composition is 80 Ni and 20 Gr., with the melting point at 1420C.

When some iron is added to the nickel-chromium alloys, it makes them more ductile. Nichrome and Chromel C are examples of these iron-containing alloys. The typical composition of Nichrome is 60 Ni, 12 Cr, 26 Fe, 2 Mn, and of Chromel C, 64 Ni, 11 Cr, 25 Fe. The melting temperatures of these alloys are 1350 and 1390C. respectively. The change of resistance with temperature for these alloys is illustrated in Fig. 12.

Thermocouple alloys. Chromel P gives a useful base-metal thermocouple in combination with the alloy Alumel (94 Ni, 2-1/2 Mn, 1/2 Fe). The thermocouple wires are welded together under a borax flux to make the junction. None of the Chromels braze, but they all may be welded to nickel.

Constantan (45 Ni, 55 Cu) has practically zero temperature coefficient of resistance up to a temperature of 400C. Also, it gives a high thermal e.m.f. against copper, making an excellent thermocouple. Constantan exhibits high resistance to oxidation and corrosion. It solders easily.

Solders. Solders are required to flow onto the surface of the metals to be joined and to alloy with the surface layers of the metals. Also, they should be ductile, have high strength, and be noncorrosive.

Silver solder best meets all these requirements. It is used for joining brass, steel, stainless steels, and many other metals. Silver solders are, in effect, brazing alloys of the composition (4 Cu to 3 Zn) with silver added. A solder melting at 693C. contains 65 per cent silver, while one melting at 760C. contains but 20 per cent silver.

High-quality soft solder is half tin and half lead. Solders are often made with a higher content of lead, since the tin component is more expensive than lead. Such solders are inferior, since it is the tin component that makes the solder run well and adhere well. "Half-and-half " solder melts at 188C. The properties of various solders are given in Table II.

Brass and Bronze. Brass is the most widely used construction material in the physical laboratory. It is fundamentally a copper-zinc alloy. Red brass (10 to 20 per cent zinc), or so-called Tombak alloy, is used for making flexible corrugated tubes (such as Silphon tubes) when maximum ductility is required; yellow or common brass, which contains copper and zinc in the proportions 65 to 35, with small lead additions to increase its machinability, is used where springiness is desired.

Brasses are less expensive than the copper-tin alloys or bronzes. They are also softer and more ductile. Brasses are used for drawing and rolling, whereas bronzes are primarily casting materials. Bronze castings are much more likely to be vacuum tight than brass castings. Also, because bronzes have small crystals of the hard brittle compound Cu₄Sn, they make good bearing metals (the 68.2 copper bronze, Cu₄Sn, is the true speculum metal used for optical gratings and for mirrors).¹⁰

Duraluminum. The aluminum alloy with composition 95 A1, 4 Cu, 1/2 Mg, 1/2 Mn, is known as Duraluminum. Duraluminum is employed extensively in many cases where brass was formerly used. For about 45 minutes after it has been heat treated at 530C. and quenched in water, Duraluminum is ductile and can be rolled, bent, or cold-worked. After this interval a copper aluminum compound is precipitated out of solid solution, and this precipitate " keys " the crystals of the alloy at their slip planes, giving the alloy increased hardness and strength. The tensile strength, originally 30,000 lbs./square inch after quenching, becomes as great as 75,000 lbs./square inch after cold-working and aging. Duraluminum rivets are frequently stored in buckets cooled with dry ice. They may be used as desired, for this low temperature arrests the aging process, and the metal does not harden until after it has warmed up to room temperature.

Wood.¹¹ Two kinds of wood are obtained from a tree: heartwood and sapwood. The heartwood is formed early in the life of the tree and, as the name implies, is found near the center of the trunk. Protoplasms present when the tree is young are gradually replaced by deposits of gum, minerals, tannin, and pigments to form this heartwood as the tree becomes older. These substances make it heavier, stronger, and in most cases darker than the sapwood. The heartwood of the redwood tree, which is particularly free from gums and oils, is an exception. In other heartwoods there are abundant deposits. For example, in lignum vitae, these compounds produce an oiliness (especially when the wood is wet) which makes it suitable as a bearing material.

Sapwood, or the outer part of the tree, is more pliable than heartwood. Therefore, in using such woods as hickory and ash, which are noted for their adequate strength, the outer part of the trunk may be preferred to the heartwood because of its pliability.

Effects of temperature. Some of the effects of temperature on wood are due to the gum deposits. High temperature softens these gums, making the wood weaker and more liable to split. On the other hand, low temperatures produce increased brittleness.

The heat conductivity of several common types of wood is given in Table IV. The conduction of heat is from two to four times as great along the grain as it is across it. The conductivity depends, in a large measure, on the moisture content. To obtain maximum heat insulation, the wood must be dry. To keep it dry, particularly if the wood is to be exposed to low temperatures, it should be coated with paraffin.

Effects of moisture. One drawback to the use of wood as a material for construction, especially for scientific apparatus, lies in the fact that its dimensions may change considerably with its change in moisture content. We may take-the shrinkage from the green to the dry condition as an index of the changes one may expect with changes in humidity and residual curing. This shrinkage (radial and tangential) for several woods is given in Table V.

Most of the shrinkage in wood is at right angles to the grain; the longitudinal shrinkage, taken from the green to the cured condition, is seldom greater than 1/10 to 1/3 per cent. (It is greater than this for some woods, particularly woods grown under strong compression. Yellow pine compression wood, for example, may shrink longitudinally as much as 2-1/2 per cent when it is cured. Redwoods also exhibit considerable longitudinal shrinkage. However, longitudinal shrinkage is negligible for most of the other woods.) This property of wood, in addition to the low thermal expansion parallel to the grain, explains why wood has been used so successfully for rulers; it may suggest other applications for wood in the laboratory.

Strength of wood. Strength and rigidity do not vary from wood to wood as much as is commonly supposed. For example, the bending strength of shagbark hickory is only 2.6 times as great as that of sugar pine, and pine is inferior to hickory in rigidity by a factor of only 1.9. Pine differs from hickory not so much in stiffness as in brittleness- pine breaks where hickory bends. Spruce, of all the common woods, has the highest strength for its weight.

The tensile strength of wood varies in different directions. Along the grain its strength is ordinarily from ten to twenty times as great as it is across the grain Also, the modulus of to their homogeneity or the degree of similarity between the physical properties of the spring and the summer growth. Also, it is desirable to have a fine grain, a quality possessed by many hardwoods, especially mahogany. Of the common softwoods, poplar and sugar pine are the most homogeneous and easiest to work.

Some wood substitutes are now available which are nearly isotropic. These are formed of bonded cellulose fibers. Although they are quite homogeneous, they are not so easily worked as wood with the plane and chisel, and nailing splits them. They can be sawed with the ordinary wood saw. Masonite is an example of such a wood substitute.¹² It comes, chiefly, in three grades, a light material which is a good heat insulator, a harder material which is suitable for making boxes for instruments, and an oil-tempered waterproof material.

Waxes and cements. The physicist uses waxes and cements to seal windows into apparatus, tubes in plates, tubes together, and so forth. He uses them also to support and fasten down lenses, prisms, and mirrors. Of all waxes available, the most useful for making improvised supports and seals is the so-called universal wax.

Universal wax. Universal wax is made from 1 part Venetian turpentine and 5 parts beeswax. It is usually, although not necessarily, colored with vermilion. It should be made up in small quantities, for it oxidizes, with the result that it becomes hard and loses its desirable properties. Old pieces may be useful if the outside oxidized layers are removed and discarded. The usefulness of this wax depends upon its adhesive and plastic properties. It is quite plastic at the slightly elevated temperature attained when the wax is worked between the fingers. When it cools, it becomes fairly rigid.

Beeswax and rosin. Beeswax and rosin compound is prepared by melting together equal parts of beeswax and rosin. Its softening point is at the temperature which just begins to feel hot (47C.) and it is liquid at 10 above this temperature. Its outstanding property is its adhesiveness to cold metal. It is not very strong, but its strength is adequate for sealing vacuum systems and for fixing apparatus, as, for example, fastening a prism to the prism table of a spectrometer. It can be applied with a brush, an eye-dropper, or the blade of a knife. To secure the best bond to cold metal, the wax should be applied smoking hot with an eye-dropper or a knife. When it has been used for sealing down a bell jar, it can be removed with a putty knife, remelted, and used over and over. The smoking temperature distills off some of the beeswax, causing the compound to become harder. It may be retempered by adding more beeswax. There are many applications for which this wax is not suitable because it shrinks a great deal on solidifying. It is best "dissolved" by a mixture of equal parts of carbon tetrachloride and ethyl alcohol.

Shellac. In its pure state, shellac in stick form is known as lapidarist's cement. It has a high tensile strength and shear strength. (Both are about 3800 lbs./square inch.) Only the natural orange shellac possesses this high strength. The main ingredient of the better grades of sealing wax and especially banker's wax is shellac.

Shellac is used in commerce chiefly for the manufacture of phonograph records, varnishes, and as an insulator in the electrical industry. It has a higher resilience than almost any other wax, and it is this property which gives long life to phonograph records.

The best solvent for shellac is alcohol. This solution yields a varnish which has many uses in the laboratory. When it is very thick, it is useful for hunting leaks in vacuum systems.

Shellac is polymerized by heat, giving a product which is harder, has a higher softening temperature, and is less soluble in alcohol than the uncured material. This polymerization is accompanied by a chemical loss of water and a two- to threefold increase in molecular weight. Half of the uncured shellac is transformed into this harder variety by heating for 30 hours at 90C.; at 150C. it is completely transformed in 3 hours. When the pure shellac is to be used as a cement, it is desirable to have it in the unpolymerized state.

Commercial shellac may legally be designated as pure although it may contain as much as 3 per cent rosin. This materially weakens it. It is possible, however, to obtain shellac which is free from this adulterant.¹³

Tempered shellac. When shellac is tempered with 20 to 40 per cent wood tar, we have a wax similar to the familiar DeKhotinsky cement. This wax is not affected by water, carbon disulphide, benzol, petroleum benzine, or turpentine. It is affected only slightly by ether, chloroform, and sulphuric, nitric, or hydrochloric acids.

When DeKhotinsky cement is heated in a flame, it emits an odor and is somewhat inflammable. A new variety of tempered shellac, which has no odor and is not so inflammable, is now sold by the Central Scientific Company under the trade name of Sealstix. Sealstix has a greater working range of temperature than pure shellac and a very high strength.

Shellac can be tempered with butyl phthalate. The resulting compound has a very low vapor pressure and is particularly suitable for high-vacuum work. It is odorless and relatively noninflammable.

Shellac can also be tempered to varying degrees with oil of cassia. About 10 per cent oil is quickly added to the molten shellac. The oil gives a compound with an agreeable odor. It is useful for many purposes when its vapor pressure is not important.

Shellac can also be tempered with amyl acetate for use when the vapor pressure of this constituent is unobjectionable. Most of this solvent evaporates when the cemented elements are maintained at an elevated temperature (80C.) for an hour or so. A mixture of 2 ounces of amyl acetate to 100 g of shellac gives a cement with a strength in excess of 2500 lbs./square inch.¹⁴

Picein. This sealing compound is characterized by low vapor pressure, plasticity at room temperature, and chemical inertness. Its low working temperature (it becomes quite plastic at 50C. and is liquid at 80C.), together with its adhesiveness, recommends it for many applications. Besides its use for sealing tubes together and repairing leaks in vacuum systems, it is also used in the optical industry. It is practically unaffected by alcohol. Picein is immune even to a short immersion in cold dichromate cleaning solution. It is dissolved by benzol and turpentine. Its insulating qualities are said to be as good as amber if it is not overheated. It comes in two grades, the second being characterized by a liquefying temperature of 105C.¹⁵

Apiezon compounds.¹⁶ Apiezon compounds are especially refined residues of paraffin oils freed from high vapor pressure constituents.

The sealing compound "Q" contains graphite. It is plastic at ordinary temperatures and has a vapor pressure of mm at room temperature, and, applied to ordinary twine, it is recommended as a packing for vacuum valves.

Apiezon wax " W " has the lowest vapor pressure of any of the waxes now available. It is necessary to heat this wax to 180C. in order to raise its vapor pressure to mm of mercury. It melts at 70C., but it can best be applied at 100C. or higher. Molten, it wets metals and glass and is quite fluid. It is fairly strong at ordinary temperatures. It is soluble in zylene.

Silver chloride. Silver chloride is recommended for seals that must hold at elevated temperatures. It melts at 455C. It is insoluble in water, alcohol, benzol, and acid. It is, however, soluble in a solution of sodium thiosulphate. Most metals and glasses are wet by fused silver chloride. It is useful for sealing optically worked windows on a discharge tube. The window, after being sealed, is cooled slowly to prevent it from cracking.

Espe and Knoll describe an enamel which they recommend for cementing optical plane parallels on a discharge tube.¹⁷ This is a mixture of clay and boracic acid, the melting point of which is 450 to 600C. It is applied, as is silver chloride, by heating both the window and discharge tube in an electric oven.

The bonding materials which we have considered above are thermoplastics. With the exception of shellac, the changes in their properties are reversible with temperature We will now treat those substances which set, which can be vulcanized, and which polymerize by the application of heat. They include the synthetic resins, rubber cements, and inorganic cements.

Synthetic resins treated of here fall into three broad divisions. These are, first, the polymerized phenol aldehydes, of which Bakelite is an example; second, the condensation products formed by polyhydric alcohols with polybasic acids (these are termed alkyd resins, of which Glyptal is an example); and third, the polymerized derivatives of methacrylic acid, of which Lucite and Plexiglas are examples.

Bakelite.¹⁸ Bakelite comes in several forms that are useful to the physicist. The properties of these vary from liquid or soluble solids in the uncured condition to stable insoluble solids in the cured condition. Bakelite in the latter condition is obtainable in the form of clear, transparent sheets, blocks, tubes, and so forth. This material is light (density, 1.27) and strong (7000 lbs./square inch), is a good electrical insulator, and is insensitive to moderate heat. In this completely polymerized form it does not melt, and it chars only at a temperature of 285C. Chemically, it is relatively inert. The completely polymerized Bakelite is unaffected by hot water, oils, greases, alcohol, acetone, benzene, dilute mineral acids, including hydrofluoric, and soap. It is practically nonhygroscopic. These properties recommend it as a material for making transparent chemical apparatus, such as burettes, pipettes, beakers, and so forth. Transparent forms of Bakelite are suitable for making models for photoelastic studies with polarized light.

Several molded and laminated products bonded with Bakelite are available commercially. These have canvas, wood fiber, asbestos, or graphite as a base. The asbestos-base material is especially heat resistant, and the graphite-base material is useful for dry bearings.

Bakelite varnishes usually consist of solutions of the unpolymerized form. After application and drying, the varnish films are transformed to the insoluble form by baking.

Bakelite cements come in the form of solids and viscous liquids. The solid form melts at about 80C. (in hot water) and is transformed by heat to a form which does not melt. The liquid forms contain a volatile solvent. It is first necessary to evaporate this by preliminary heating of 1 to 4 hours at 80C., after which the residue is polymerized by heating for 2 hours at 120C. A self-hardening cement is available which will set at room temperature. Vacuum seals made with these cements have a low vapor pressure and can be used to temperatures slightly above 100C.

A general-utility cement can be made by mixing Bakelite varnish with red lead. This hardens rapidly and will withstand high pressure, steam, oil, and moderate heating.

Alkyd resins.¹⁹ Alkyd resins are formed by the condensation of phthalic anhydride on glycol, glycerol, or other polyhydric alcohols. Glycol phthalate is useful as a vacuum-sealing cement because of its low vapor pressure, fluidity, and wetting power when melted. In addition, it may be cured to give it increased strength and inertness. It is also noted for its adhesiveness to aluminum. It is inert toward mineral oils. Dehydrating catalysts, such as zinc oxide, hasten the cure of these compounds and serve as a filler to economize on the resin, as, for example, in lamp-basing cements.

Lucite and Plexiglas. Lucite and Plexiglas are trade names for methyl (and ethyl) methacrylate, polymerized derivatives of methacrylic acid.²⁰ These materials are sold as a cast resin in the form of sheets, rods, and tubes, as a thermoplastic powder, and as the unpolymerized liquid.

The methyl methacrylate monomer is a mobile liquid which can be polymerized in almost any desired form. The monomer boils at 100C. and has a heat of polymerization of about 80 calories/g. As it is obtained from the factory, it contains an inhibitor, such as hydroquinone or pyrogallol, to prevent it from polymerizing at room temperature. To use the liquid, this inhibitor is removed by washing with caustic, the liquid is dried, and an accelerator, usually benzoyl peroxide, is added to catalyze the polymerization. The volume of the monomer is 20 per cent greater than the volume of the polymer finally obtained, so that considerable art must be invoked to prevent the formation of voids when the monomer condenses.

These polymers are vastly different from Bakelite and the plastic, Catalin, in respect to cutting. Whereas Catalin and Bakelite quickly remove the edge from high-speed steel (in fact, cold-rolled steel is just about as good for cutting them as high-speed steel), Lucite and Plexiglas can be cut by the hour without the edge of the tool becoming dulled.

Fish-glue cement. A cement which is inert toward most organic solvents is made from a thick solution of 3 ounces of fish glue, 1/4 ounce of potassium bichromate, and a little ammonia. The cement so formed is allowed to dry and is then heated in an air oven until it assumes a chocolate-brown color. This cement is often used on Pulfrich refractometers.

Rubber cements. Rubber cements are conveniently classified as follows: nonvulcanizing cements, which attain their strength simply by the evaporation of a solvent; vulcanizing cements, in which a chemical change occurs after the evaporation of the solvent; and thermoplastic cements. Some of the vulcanizing cements contain sulphur, while others are vulcanized simply by painting a vulcanizing liquid, sulphur chloride, on the rubber after it has been applied.

The synthetic thermoplastic rubber-like products Neoprene (manufactured by DuPont Company) and Koroseal (manufactured by the Goodrich Rubber Company) have many useful properties. These materials are remarkably stable chemically; they are inert toward acids and alkalies, E as well as many fats and oils.

Plaster of Paris. This is frequently used to support large glass bulbs containing mercury. The plaster suspended in water to the consistency of a paste is cast between the bulb and a loose-fitting wooden support. Salt shortens the time required for plaster of Paris to set, while a trace of glue acts in the opposite way. The glass may first be wiped with oil so that the plaster will not adhere to it. This facilitates subsequent dismantling.

Litharge and glycerin. This combination gives a cement useful for the same type of applications for which plaster of Paris is useful. It is inert toward water, most acids, and all alkalies, and holds up to temperatures of 260C. It is prepared by mixing pulverized Litharge (which has been first thoroughly heated at 400C.) with pure glycerin to the consistency of a paste.²¹

Other irreversible cements. Water glass forms cements when mixed with the carbonates or oxides of calcium, magnesium, zinc, lead, or iron. In a few hours these mixtures set to rock hardness. Combined with talc, water glass makes a cement which holds even at a red heat. This cement will not chip off from glass at liquid air temperatures.

Zinc oxychloride cement is used extensively in dentistry. It is formed from a 60 per cent zinc chloride solution and zinc oxide powder mixed to the consistency of a thick paste. These constituents react to give zinc oxychloride. To insure that the oxide is free from carbonate, it should first be heated until it turns yellow to calcine the carbonates.

Nine parts kaolin mixed with one part borax give a cement useful to 1600C. The constituent powders are mixed, and water is added to facilitate application. After the water evaporates, the cement is slowly heated to a yellow heat in order to set it.

Insa-lute cement, a commercial product, is a thick white suspension of refractory substance in water glass. It sets on drying to form a white material having the texture of porcelain. It is an electrical insulator and stands firing to about 1100F. It adheres to metal, glass, and porcelain. It attacks chromium-alloy wire at elevated temperatures and should not be used in contact with it. One should use a refractory cement such as Alundum cement in contact with chromium-alloy wires.

Glue. Unquestionably, the best bonding material for wood is glue. Glues are more effective for the lighter woods, which contain less oils and resins, than for the dense woods. There are three kinds of glue: casein, blood albumin, and animal glues. The first two are useful for general construction work. The animal glues exhibit greater strength, but they are softened by moisture. Casein glue is made from milk protein and lime. Blood glues contain caustic soda and water glass. Both the latter glues require heat and pressure for their application. They are water resistant and are used for making plywood.

Animal glue is best applied hot. Cabinet and pattern makers usually keep a hot glue pot. For occasional use, however, air-drying glues are quite satisfactory for joining wood as well as leather. Air-drying glue is applied to the surfaces which are to be fastened together. The glue films on these surfaces are allowed to dry until the glue is definitely stringy. At this stage the surfaces are clamped together, and the glue is allowed to become completely dry.

Lubrication. There are two kinds of lubrication with liquids. In the first and most common kind, called complete lubrication, the bearing surfaces are separated by a layer of oil about .005 inch in thickness. The friction, and consequently the amount of heat produced in the lubricant, depend on the thickness and viscosity of the liquid.

In the second kind of lubrication the surfaces are in contact. Friction and galling are diminished and prevented by an absorbed surface film. The tenacity with which this film adheres and the effectiveness with which it reduces friction are determined by a quality called the oiliness of the liquid. The lubrication of surfaces in contact is called boundary lubrication. In general, mineral oils are more suitable for the first type of lubrication, and vegetable and animal fats, as well as soaps, are more suitable for the boundary lubrication. The friction in the case of boundary lubrication is usually less when the surfaces are covered with a surface film of high molecular weight. High viscosity (which is associated with high molecular weight) gives more friction in the case of complete lubrication.

Sir Wm. B. Hardy and Miss Ida Doubleday²² have studied the coefficient of boundary friction for various materials, and the results of their study are illustrated by Fig. 13.

The boundary friction is represented in this figure for glass rubbing on glass, steel on steel, and bismuth on bismuth. The coefficient of friction is plotted as ordinate against the molecular weight of the lubricant as abscissa. The paraffins and the alcohols and acids of the paraffin series are the liquids used as lubricants.

We see that for very high molecular weights, particularly in the case of the fatty acids, the coefficient of friction is expected to be zero, and, indeed, for some compounds Hardy found the static friction to be less than the minimum amount that he could measure.

Of these results, Hardy and Doubleday say:

It will be seen that for each chemical series, and for each solid, the curve is a straight line. The equation is therefore

where M is molecular weight and a and b are parameters. The effect of the nature of the solid face is unexpectedly simple. In changing from glass to steel the curve for a series is merely moved parallel to itself, and in changing from steel to bismuth there is a further shifting. Therefore, in the equation the parameter a is independent of the nature of the solid face and dependent only on chemical type, varying from one chemical series to another. The parameter b, on the other hand, is dependent upon the nature of the solid face as well as upon the chemical series.

From the above expression one might expect the coefficient of friction for two dissimilar surfaces to be a mean of the coefficient for the two separate surfaces, and Table VIII, from the paper of Hardy and Doubleday, shows how well this expectation is realized.

The addition of a small amount of fatty acid to a mineral oil materially improves lubrication, especially where the bearing surfaces come in contact and the character of lubrication changes from the complete type to the boundary type. The higher efficiency of boundary lubrication obtained in this way is due to the adsorption of a fatty acid film of high molecular weight on the bearing surfaces.

The suitability of a lubricant for use in a scientific instrument often depends primarily upon the ability of the lubricant to form stable films which cover the surface of the metal and produce a diminution of static friction between sliding metal parts. Oils which are rich in vegetable or animal fats are superior to petroleum oils in this regard. Hydrated lanolin is an excellent lubricant for sliding friction heads in which a low starting (or static) friction is important. Unfortunately, it is somewhat corrosive.

Colloidal graphite added to oil is similarly adsorbed on bearing surfaces. It is useful for the lubrication of spectrometer cones and, in machinery, it is especially useful for the running-in operations to form a hard polished Beilby layer on the bearing surfaces.²³

For extremely heavy-duty lubrication, mutton fat or Dutch grease is recommended. Dutch grease is simply a combination of mutton fat with heavy petroleum oils.

It is desirable for clocks and other delicate mechanisms to have an oil that is chemically stable, does not corrode the metal parts, does not escape by spreading or evaporating, and does not freeze easily.

The oils from the head and jaw of the porpoise (Nye watch oil) and blackfish satisfy these qualifications most completely. These oils, however, are quite expensive ($125.00 a gallon). Sperm oil is next in quality.

Soap is a good lubricant for wood. Water is used between rubber and metal surfaces. Talc is often used as a dry lubricant for nailing and so forth. Also graphite, especially colloidal graphite, forms a good dry lubricant at ordinary and elevated temperatures. It is used for lubricating lock barrels. Colloidal graphite dispersed in water or glycerin has film-forming properties which recommend it for some applications. The glycerin dispersion is useful at low temperatures.

Soapstone.²⁴ When soapstone, or massive talc, is heated to 500C., it gives off absorbed water. Heating it to 850C. drives off the remaining water, and finally heating to 1300C. gives a complete transformation of the mineral constituents. When the material is transformed, the corresponding overall change of hardness is from 1 to 6 on Moh's scale. Owing to these properties, soapstone is very useful in the laboratory, since it can be easily machined before it is fired, and after it is fired it gives a hard material having many desirable properties. The shrinkage from firing is less than 1 per cent. Firing from 24 to 48 hours at 1100C. is the usual practice. It is said that a material with iron content is not suitable for use in a vacuum, since it gives off gas. Imported Italian soapstone is exceptionally free from iron impurities and is obtainable in large blocks.²⁵

Finely divided talc can be pressed and fired at 1400C., giving a sintered body like that obtained from the massive mineral. The shrinkage of the powder on firing is about 8 per cent. After firing, it has the property that it is not attacked by acids or alkalies. It can be welded to glass.

Talc, both fired and unfired, is chemically inert; it is not attacked by acids (except slowly by hydrochloric acid) or by alkalies. Fig. 14 shows the electrical resistance of talc and 15 some other refractory materials as a function of temperature.