SELENIUM ) a nonmetallic. solid, elementary body, discovered in ISIS by Berzelius in the iron pyrites of Fahlun ; the sulphur pro cured from which was used at Gripaholin In the manufacture of sul phuric add. In the latter a deposit was formed of a red colour, which, en account of the peculiar odour that It emitted, was supposed, though erroneously, to contain tellurium, the origin of which name suggested that of selenium, from :Emirs, the moon, for the new substance. The discoverer considered it a metal, but it is now classed with the non metallic elements.
Selenium has been found hitherto only in small quantity ; it occurs in the pyrites of Anglesey, and probably in that of many other places : sulphide of selenium has been detected among the volcanic products of the Lipari Isles ; and in the Harz it has been met with, combined with lead, silver, and some other metals.
3Isgnua extracts selenium from the native sulphide by treating it with binoxide of manganese, by the oxygen of which the sulphur is converted into sulphurous acid, which escapes in the gaseous form, while the selenium either sublimes in its pure state or as selenious acid.
Selenium is a brown somewhat transcluceut solid when in mass. It is inodorous and insipid, moderately bard, may be readily scratched with a knife, is brittle as glass, and easily reduced to powder. Its fracture is conchoidal. Its sp. gr. varies from 4-30 to 4-5, on account of the cavities which it frequently contains. It is a bad conductor of beat, and a non-conductor of electricity. It softens at 212°, and may be drawn out into fine threads, which are transparent, and of a red colour by transmitted light. When heated rather higher, it becomes fluid, and boils at 650', emitting a vapour, which is inodorous, and of a deep yellow colour ; this in close vessels condenses in dark globules of a metallic lustre, or of a cinnabar-red colour, according as the space in which it collects is small or large. Water does not dissolve selenium ; it is however soluble in the fat oils and incited was, but not in the volatile oils. The equivalent of selenium is 39.75.
Oxygen and Sdenium combine in three different proportions, form ing oxide of selenium, selenious acid, and selenic acid.
Oxide of Selenium (SeO) may be formed by heating the selenium in a limited quantity of atmospheric air, and by washing the product to separate the selenious acid formed with it. It emits a very strong smell resembling that of decayed horse-radish, so that 1.50th of a grain of selenium is sufficient when burnt to scent a room of consider able size, and this is a characteristic property of selenium. Oxide of selenium is gaseous, colourless, very slightly soluble in water, and quite devoid of acid properties.
acid may be prepared by passing s. current of oxygen gas over heated selenium ; but it is more conveniently obtained by digesting selenium in nitric acid or nitro-hydrochloric acid till dis solved, and then evaporating the solution to dryness.
This acid is colourless, and when strongly heated sublimes, and con denses unchanged in the form of acieular crystals, which possess distinctly marked acid properties. It attracts moisture from the air and is consequently very soluble in water; a hot saturated solution yields crystals on cooling ; it is also soluble in alcohol, and has when heated an acrid odour. It forma salts with bases, which are called &knifes.
Many substances which have strong affinity for oxygen decompose %denleu, acid ; this is the case with sulphurous acid and phosphorous acid, and when the former is added to a solution of selenious acid, a red powder is precipitated, which is pure selenium, and sulphuric acid is at the same time formed. An alkaline sulphate produces a similar effect ; hydrosulphuric acid is also decomposed by and decomposes selenious acid, and a yellow compound is formed and precipitated, which is sulphide of selenium.
Selenie Arid (110.Se0,) may be prepared by fusing selenium, a Bele nide, or selenite, with nitrate of itoda. The edentate of soda obtained Is to be decomposed by nitrate of lead, and the insoluble seleniate of lead precipitated is to be decomposed by a current of hydrosulphuric acid, which throws down the lead as a sulphide. The selenie acid remains in solution, with some excess of hydrolulphuric acid, which is to be expelled by ebullition.