In 1861, thallium was discovered by accident in a residue of a sample that had been used to study tellurium and selenium. Rather than discarding the residue, it was studied with a spectroscope. Instead of seeing the blue selenium lines and the yellow tellurium lines, a brilliant green line was observed. The element was named thallium, meaning green branch.

Thallium is very much like lead in appearance. But unlike lead, thallium reacts fairly rapidly with oxygen and moisture in the air. Thallium is found in lead and cadmium ores.

Thallium's uses in metallic form are chiefly in alloys with bismuth, cadmium, and mercury. Since it is quite poisonous, its compounds (i.e. thallium(I) sulfate (Tl2SO4)) have been used in rodenticides and various insecticides. It also has applications in photocells, infrared detectors, and low-melting glasses. Mixture of thallium(I) iodide, TlI, and thallium(I) bromide, TlBr, are used in the manufacture of glass for special infrared lenses.

BCIT Chemistry Resource Center
http://nobel.scas.bcit.ca/resource/