Tellurium (Lat. tellus, the earth), an elementary substance, discovered by Muller von Reichenstein in 1782, but first investigated and named by Klaproth in 1798; symbol, Te; chemical equivalent, 129; specific gravity, 6.65; hardness, 2 to 2.5. Though commonly classed among the metals, it has much analogy in its properties to sulphur and selenium. It fuses between 800° and 900° F., and can be distilled in a current of hydrogen. It is a bad conductor of heat and electricity. It occurs in a native state associated with iron pyrites and various metals, as gold, silver, bismuth, copper, or lead. The native metal is of a brilliant metallic lustre, of a tin-gray or lead-gray color, passing to steel-gray. It is very fusible before the blowpipe, and burns with a bluish flame, green on the edges; it volatilizes in white fumes, leaving no residue; and it is wholly soluble in nitric acid. The substance occurs in small masses, irregularly lamellar, and crystallized in six-sided prisms, at the mine of Maria Loretto near Zalatna in Transylvania. Its most common ore is the black, foliated mineral of Nagyag, which contains about 13 per cent. of tellurium in the form of tellurides of gold, lead, and silver, mixed with sulphides of antimony and lead.

Tellurium is almost always combined with small portions of iron or gold in a metallic state, silver, or lead, so that some have supposed that the substance ought to be considered as telluride of iron or of gold. Many natural alloys have been met with at the mines of Hungary and Transylvania, and from the collection of those presented by the emperor of Austria to the museum of natural history at Paris, Dufrenoy has arranged the varieties among the following five species: native tellurium, auro-argentiferous tellurium (graphic gold), auro-plumbiferous tellurium (mullerite), plumbo-auriferous tellurium (nagy-agite), and telluric bismuth (tetradymite). Auro-argentiferous tellurium was recognized at the Gold Hill mines, North Carolina, and native tellurium at lied Cloud mine, Gold Hill, Boulder co., Colorado, by Dr. Genth; and telluric bismuth is found in many of the gold mines of Virginia and North Carolina, in foliated scales and lamellar masses. Gold and silver tellurides occur in masses on the Calaveras range in California. Tellurium forms two oxides, TeOo, Te03, which correspond in composition to sulphurous and sulphuric anhydrides. Tellurous acid, H2Te03, and telluric acid, HoTeO-i, are analogous to sulphurous and sulphuric acids.

With hydrogen it forms the gaseous compound H2Te, analogous to sulphuretted hydrogen.