Thallium

Thallium (T1 = 204), a metallic element which was first discovered by Crookes in 1861, when examining the spectrum of some dust deposits from sulphuric acid chambers. It has since been shown to be found in many varieties of pyrites, in numerous mineral waters, and a few minerals. The metal itself is very soft, and can be scratched with the finger nail. It may be drawn into wire and beaten into foil. It has a bluish-white colour and is distinctly crystalline, a bar of the metal emitting a peculiar crackling sound if bent. It melts at about 300° C., and readily oxidises, and, if heated sufficiently, may burn with a bright green light. It forms a well-defined series of thallious salts corresponding to the oxide Tl₂O. Of these the chloride is almost insoluble like that of lead, differing, however, in the ready solubility of the sulphate.