Selenium

Selenium (SE,79.9), a rare non-metallic element, occurs to a small extent in certain iron pyrites, and, owing to its presence in this source, sometimes in deposits in the lead chambers used in the manufacture of sulphuric acid (q.v.). It was in these deposits that the element was first discovered by Berzelius in 1817. In most of its chemical properties selenium resembles sulphur, and is capable of existing in two allotropic forms. One of these is obtained as a reddish-brown powder by the reduction of the oxide of selenium, and is soluble in carbon disulphide. If melted and kept molten for some time, being allowed to cool very slowly, a crystalline variety is obtained. This form is insoluable in carbon disulphide, has a specific gravity of 4.6, and melts at 217° C. If strongly heated in air selenium burns, forming a dioxide SeO₂, with a strong and peculiar odour. This oxide condenses to form needle-shaped crystals, and disolves in water to form selenious acid, from which, by oxidation, a powerful acid (selenic acid) results. This acid is remarkable as being the only single acid which is capable of dissolving gold. It forms salts, known as the selenates, which in most points closely resemble the sulphates. With hydrogen, selenium forms a componnd, SeH₂, which is a combusbtible colourless gas with a most penetrating and disagreeable odor, powerfully attacking the throat and eyes. It is easily soluble, and resembles the corresponding sulphur compound, producing, in solution of most metallic salts, precipitates of metallic selenides. It. was so-called from selene, "the moon," because in many respects it resembled tellurium (from tellus, " the earth ").