THALLIUM (symb. Tl, equiv. 204, spec. gray. 11.9) is a metal which derives its name from the Greek word thallos, green, because its existence was first recognized by an intense green lite appearing lu the spectrum of a flame in which thallium is volatilized. It was discovered by Mr. Crookes, the editor of the Chemical News, in 1861, in the seleniferous deposit of a lead chamber of a sulphuric acid factory in the ,Hartz moun tains, where iron pyrites is employed for the production of the acid. In ,the following year, it was obtained in larger quantities from a similar source by M. Lamy, who exhib ited magnificent specimens of it in solid bars at the last great exhibition in London. Thallium is slightly heavier than leada metal which it resembles in its physical prop erties. It is very soft, being readily cut with a knife, or drawn into wire; and its freshly-cut surface exhibits a brilliant metallic luster and grayish color, somewhat between those of silver and lead. In contact with the air, it tarnishes more rapidly than lead, and becomes coated with a thin layer of oxide, which preserves the rest of the metal. It fuses below a red heat, and is soluble in the ordinary mineral acids. With oxygen, it enters into two combinations—viz., Oxide of Thallium, which is a strong
base, forming well-defined salts with acids; and Thallic Acid, which is soluble in water, maybe obtained in crystals, and forms soluble salts with the alkalies.
There is a difference of opinion as to whether the salts of thallium are or are not powerful irritant poisons. Lamy (with the view of testing the statement of Paulet, that the salts are poisonous) dissolved 75 grains of the sulphate in milk; and he found that this quantity sufficed to destroy two hens, six ducks, two puppies, and a middle sized bitch. In one experiment, a grain and a half proved fatal to a puppy. Mr. Crookes, on the other hand, although much exposed to the fumes of this metal, suffered no particular effects from them; and he swallowed a grain or two of the salts without injury. He found that the latter have a local action on the hair and skin, staining the former, and rendering the latter yellow and horny.—For further details regarding this metal, the reader may consult Mr. Crookes's memoirs in the Philosophical Transactions for 1862, and in the Chemical News, and Lamy's memoir in the Annales de Chimie et de Physique for 1863.