Osmium

Osmium (sym. Os; atom. wt. 191) is a metal which occurs in association with platinum in the form of an osmium-iridium alloy. It may be obtained in the metallic condition by several processes which yield it either as a black amorphous powder or in hard bluish-white crystals. It is the least fusible of all the metals, the oxyhydrogen jet volatilising, but not fusing it. It is the heaviest substance known, its specific gravity being 22.477. Four oxides of osmium are known. Three of these—viz. the protoxide, $\backslash text\{OsO\}$, the sesquioxide, $\backslash text\{Os\}\_2\backslash text\{O\}\_3$, and the binoxide, $\backslash text\{OsO\}\_2$, are black or grayish-black powders. The peroxide, $\backslash text\{OsO\}\_4$, commonly called osmic acid, is the most important oxide. It is produced when the metal is heated strongly in air or oxygen, and forms colourless, glistening, acicular crystals, freely soluble in water, and very volatile. At about $100^\backslash circ\backslash text\{C\}$. this compound gives off an extremely irritating and irrespirable vapour; and hence the name of the metal (from the Greek word osmē, 'odour'). It produces a permanent black stain upon the skin, and at the same time causes an eruption which is difficult to heal. It violently attacks all the mucous membranes, and its vapour may cause partial or total blindness by depositing a film of metallic osmium on the eyes. A solution of the peroxide is employed in histological work for staining fat and nerve substance. Osmium also forms two chlorides; and osmates, corresponding to an unknown osmic acid, have been prepared. This metal was discovered by Tennant in 1803.