Thallium

a metallic element akin to lead and zinc. It was discovered in 1861 by Sir William Crookes in the lead chambers of sulphuric acid works in the Harz Mountains. He detected its presence by a brilliant green line shown in the spectroscope. The name is Greek, signifying a green twig. Before dross is formed on the surface, thallium presents a silvery white luster. It is softer than lead and, like it, is malleable. It melts readily. Its chief use in the arts is in the making of glass. A trace of thallium in the lens of a microscope increases its magnifying power. Thallium is also used to add luster in the manufacture of glass jewelry.