Thallium (Gr. θαλλός, a green bough), one of the three metals forming the class of triads, the others being iridium and gold. It was discovered by Crookes of London in 1861, in the seleniferous residue from the manufacture of sulphuric acid from iron pyrites. Its discovery resulted from the observation by Mr. Crookes of a green band in the spectrum of the vaporized portion of the residue. It is widely diffused as a constituent of iron and copper pyrites, but forms only about the 4,000th part of the mass. It also exists in the lepidolite of Moravia, in mica from Zinnwald in Bohemia, in the mother liquors of the salt works at Nauheim, and in the mineral crookesite from Skrikerum in Norway. It is most economically prepared from the flue dust of pyrites burners. This dust is stirred with boiling water in wooden tubs, and the decanted or syphoned liquor treated with an excess of strong hydrochloric acid, by which impure monochloride of thallium is precipitated. This impure chloride is then treated with hot oil of vitriol, and contaminations of other metals are separated by sulphuretted hydrogen. A pure sulphate is obtained, from which the metal may be separated by electrolysis or the action of zinc.

Thallium resembles cadmium in color, but approaches lead in specific gravity, having a density of 11.8 to 11.91, according to its metallurgic treatment. The symbol of thallium is Tl; its atomic weight, according to recent extended researches by its discoverer, is 203.642. (See " Chemical News," London, 1874.) It has a highly crystalline structure, and crackles like tin when bent, but is easily hammered into leaves. It melts at 561° F. A polished piece of the metal tarnishes rapidly when exposed to the air, but the action continues only a short time, as the thin film of oxide protects it from further oxidation. The metal and its compounds impart an intense green color to colorless flames, which when viewed by the spectroscope is found to be monochromatic, appearing as a sharply defined green band. It forms numerous compounds, including three oxides, the most important being thallous oxide, T120; this dissolves readily in water, producing a caustic alkaline solution which absorbs carbonic acid from the air. The sulphate forms with alu-minic sulphate an octahedral alum. The salts of thallium are poisonous.

The metal has been used to render glass highly refractive.