From the CRC handbook of Chemistry and Physics, CRC Press, Cleveland, 55th ed., 1974-75 "The Elements" by C.R. Hammond. page B-33
THALLIUM (Gr. thallos, a green shoot or twig), Tl; at. wt. 204.37; at. no. 81; m.p. 303.5 C; b.p. 1457+/-10 C; sp.gr. 11.85 (20 C); valence 1 or 3. Discovered spectroscopically in 1861 by Crookes. The element was named after the beautiful green spectral line, which identified the element. The metal was isolated both by Crookes and Lamy in 1862 about the same time. Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from roasting of this ore in connection with the production of sulfuric acid. It is also obtained from the smeling of lead and zinc ores. Extraction is somewhat complex and depends on the source of the thallium. When freshly exposed to air thallium exhibits a metallic luster, but soon develops a bluish-gray tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife. Twenty isotopic forms of thallium, with atomic masses ranging from 191 to 210 are recognized. Natural thallium is a mixture of two isotopes. The element and its compounds are toxic and should be carefully handled. Contact of the metal with the skin is dangerous, and when melting the metal, adequate ventilation should be provided. The maximum allowable concentration of soluble thallium compounds in air is 0.1 mg/cu meter. Thallium sulfate is widely employed as a rodenticide and ant killer. It is odorless and tasteless, giving no warning of its presence. The electrical conductivity of thallium sulfide changes with exposure to infrared ligth and this compound is used inphotocells. Thallium bromide-iodide crystals have been used as infrared detectors. THallium has been used, with sulfur or selenium and arsenic, to produce low melting gasses which become fluid between 124 and 150 C. These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water. Thallium oxide has been used to produce glasses with a high index of refraction. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60 C, some 20 C below the freezing point of mercury. Commercial thallium metal costs about $8/lb. It is available also in high-purity form at a cost of about $4/oz.
