THALLIUM, a metallic chemical element, discovered in 1861 by Sir William Crookes, who, during a spectroscopic examination of the flue-dust produced in the roasting of seleniferous pyrites occurring at Tilkerode in the Harz, observed a green line foreign to all then known spectra. He concluded that the mineral con tamed a new element, to which he gave the name of thallium, from OaXX6c, a green twig. Crookes presumed that his thallium was something of the order of sulphur, selenium or tellurium; but Lamy, who anticipated him in isolating the new element, found it to be a metal. (Symbol Tl, atomic number 81, atomic weight 204.3.) Chemical Character.—The chemical character of thallium presents striking peculiarities. Dumas once called it the "Ornitho rhynchus paradoxes of metals." As an elementary substance, it is very similar in its physical properties to lead; it resembles lead chemically inasmuch as it forms an almost insoluble chloride and an insoluble iodide. But the hydroxide of thallium, in most of its properties, comes very close to the alkali metals ; it is strongly basic, forms an insoluble chloroplatinate (platinichloride), and an alum strikingly similar to the corresponding potassium compounds. Yet, unlike potassium or lead, it forms a feebly basic sesquioxide similar to manganic oxide, Traces of thallium exist in many kinds of pyrites, as used for vitriol-making. The only known mineral of which it forms an essential component is the rare mineral crookesite of Skrikerum, Sweden, which, according to Nordenskiiild, contains 33.3% of selenium, 45.8% of copper, 3.7% of silver, and 17.2% of thallium. The best raw materials for the preparation of thallium are the flue-dusts produced industrially in the roasting of thalliferous pyrites and the "chamber muds" accumulating in vitriol-chambers worked with such pyrites; in both it is frequently associated with selenium. The flue-dusts may contain 0.5-3.5% of thallium, but usually do not exceed the lower value.
For the extraction of the metal from chamber mud, the latter is boiled with water, which extracts thallium as sulphate. From
the filtered solution the thallium is precipitated as chloride by addition of hydrochloric acid, along with more or less of lead chloride. The mixed chlorides are boiled down to dryness with sulphuric acid to convert them into sulphates, which are then separated by boiling water, which dissolves only the thallium salt. From the filtered solution the thallium is recovered, as such, by pure metallic zinc, or by electrolysis. The (approximately pure) metallic sponge obtained is washed, made compact by compres sion, fused in a porcelain crucible in an atmosphere of hydrogen, and cast into sticks.
Metallic Thallium.—Metallic thallium is bluish white; it is extremely soft and almost devoid of tenacity and elasticity. Its specific gravity is 11.86. It fuses at 302° C; at about 1,300° it boils and can be distilled in hydrogen gas. Its vapour density corresponds to monatomic molecules. Its salts colour the Bunsen flame a bright green. When heated in air it is readily oxidized, with the formation of a reddish or violet vapour. When exposed to the air it becomes quickly covered with a film of oxide ; the tarnished metal when plunged into water reassumes its metallic lustre, the oxide film being quickly dissolved. When kept in con tact with water and air it is gradually converted into hydroxide, T1OH. It decomposes water at a red heat, liberating hydrogen and being itself converted into the hydroxide. It is readily soluble in nitric and sulphuric acids, but less so in hydrochloric. It does not combine with hydrogen, but unites directly with the halogens.
Salts.—Thallium forms two series of salts: thallous, in which the metal is univalent; and thallic, in which it is trivalent. The thallic salts are, however, much less stable than the thallous. In the thallous series many analogies with lead compounds are ob served ; in the thallic some resemblance to aluminium and gold.