Thallium

Thallium (syn. Tl, equiv. 203.6) is a metal which derives its name from the Greek word thallos, 'green,' because its existence was first recognised by an intense green line appearing in the spectrum of a flame in which thallium is volatilised. It was discovered by Mr Crookes in 1861 in the seleniferous deposit of a lead chamber of a sulphuric-acid factory in the Harz Mountains; and it was soon obtained in large quantities by M. Lamy. Thallium is slightly heavier than lead—a metal which it resembles in its physical properties. It is very soft, being readily cut with a knife or drawn into wire; and its freshly-cut surface exhibits a brilliant metallic lustre and grayish colour, 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 at 554° (290° C.), and at a red heat becomes volatilised. The metal and its compounds give a bright green tint to colourless flames; the spectrum of thallium is marked by a single sharply-defined green line. It is used to produce a green light in firework displays, and is employed to render glass highly refractive. The metal can best be preserved free from oxidation by being covered with paraffin and kept below water. Thallium forms a number of compounds, including three oxides.