Selenium (sym. Se; at. wt. 79) is an element having two forms. In the vitreous form, at ordinary temperatures, it is a solid of a dark-brown colour, and when broken presents a conchoidal vitreous fracture; thin splinters of it are, however, of a dark-red tint when seen by transmitted light. It is tasteless and inodorous, a non-conductor of electricity. Its specific gravity is 4.28; its melting-point is 217° C., and its boiling-point 700° C. When selenium is very slowly cooled from the fused condition its appearance is quite different; the structure being granular or crystalline (sometimes called 'metallic'). Crystalline selenium is of a dull leaden colour; it is very opaque to light even in thin films; its specific gravity is 4.8, its melting-point 200° C., and its boiling-point 680° C. It is a conductor of electricity at ordinary temperatures. Its resistance to the passage of an electric current diminishes up to the point of fusion, but suddenly increases as the selenium becomes liquid. Another property of crystalline selenium, which has recently given it a new interest, is that it is remarkably sensitive to light; and its electrical resistance varies very much according to its exposure to light, being much less in the light than in the dark. It was in virtue of this property that the experiments were made which led in 1880 to the discovery of the Photophone (q.v.). The vapour of selenium is inodorous and deep yellow; it is 164 times as heavy as hydrogen at 1400° C.; this corresponds nearly to the molecular formula . When heated in the air selenium does not very readily take fire; but it is combustible, and burns with a blue flame, while a portion of it is volatilised in red fumes. The products of combustion are oxide of selenium and selenious anhydride, .
Selenium is of rare occurrence in nature; it is chiefly found as a selenide in combination with lead, silver, copper, or iron; but it has also been discovered in sulphur, and in certain sulphides of iron. It forms with oxygen a suboxide and a binoxide (, selenious anhydride); and as it forms also a selenic acid, aq, the existence of a selenic anhydride is inferred, though it has not been isolated. With hydrogen selenium forms seleniuretated hydrogen, or hydro-selenic acid, . Selenium was discovered in 1817 by Berzelius, in the refuse of a sulphuric-acid manufactory. He named it selenium (Gr. selênē, 'the moon'), because in many respects it resembled tellurium (from Lat. tellus, 'the earth').