Tellurium (sym. Te, equiv. 128) is a chemical element, which some authorities place among the metals, and others among the non-metallic bodies or metalloids. Although in its outward characters it closely resembles the metals, its close analogies with sulphur and selenium indicate that its true place is amongst the metalloids. It possesses a high metallic lustre, and is bluish white in colour; it melts at about 932° (500° C.), and at a higher temperature is converted into a yellow vapour; it is a bad conductor of heat and electricity, and is very brittle. Its specific gravity is 6.24. When strongly heated in the air it burns with a blue flame and gives off white fumes of tellurous acid. Like sulphur and selenium, it is soluble in cold oil of vitriol, to which it gives a fine purple-red colour, and on dilution it is precipitated unchanged; in these respects it differs from all metals. In nitric acid it dissolves with oxidation.
Tellurium forms two compounds with oxygen—viz. Tellurous acid, , and Telluric acid, , corresponding to the oxides of sulphur. Tellurous acid exhibits very slight acid properties, and in the anhydrous state it combines with acids, and acts the part of a weak base. These salts have a metallic taste, and are said to act powerfully as emetics. The telluric acid has only a feeble affinity for bases, but it forms salts, which contain 1, 2, and 4 molecules of the acid to each molecule of base. Tellurium unites with hydrogen to form telluretted hydrogen, , which is a gaseous body, analogous to sulphuretted hydrogen, and precipitates most of the metals from their solutions in the form of tellurides, which have a close analogy with the corresponding sulphides.
In experimenting upon the action of the salts of tellurium, it has been found that they possess the power of forming, in the body of a healthy person, compounds which impart to the breath, to the perspiration, and to the gases generated in the intestinal canal a disgusting fetor, which makes him a nuisance to every one he approaches; and this smell may last for weeks, although the quantity of tellurium that was administered did not exceed a quarter of a grain.
Tellurium is a rare substance, found chiefly in Transylvania, but discovered in other parts of Hungary, in North America, and in the Altai silver mines. It sometimes occurs native, but more commonly as a telluride of gold, lead, or silver.