OSMIUM (Os). From ocrpu, odour, in allusion to the smell of osmic acid. This metal was discovered by Mr. Smithson Tennant in the year 1803, in the grains of native platinum [Pistarsum], along with another metal [IRIDIUM]. This compound, called °amide of iridium, occurs in flat grains and hexagonal crystals. The primary form is a rhomboid. Cleavage, perpendicular to the axis. Hardness to 5.1, exceeding that of native platinum. Colour iron-gray and pale steel-gray. Lustre metallic. Opaque. Specific gravity Insoluble even in aqua regia; becomes black when fused with nitre, and when heated in a tube exhales a peculiar odour.
Berzelius analysed three varieties of onside of iridium, from Siberia, with the annexed results.
The preparation and properties of osmium have lately been studied by MM. Deville and Debray. These chemists have obtained it in the pulverulent, crystallised, and compact forms, by the following pro cesses. Osmide of iridium powdered, if necessary, by roasting with zinc, is mixed with five and a half times its weight of binoxide of barium and heated in a well covered crucible to the temperature of melting silver, for two hours. The resulting black mass is then broken up, distilled in a retort with eight parts of hydrochloric acid and one of nitric acid, the well cooled distillate redistilled, and the vapours con densed in dilute ammonia. The osiniate of ammonia solution thus formed is then saturated with sulphuretted hydrogen and boiled, and the sulphide of osmium reduced by heating for four or five hours, in a carbon crucible, to the temperature of melted nickel or melted rhodium.
Osmium thus obtained is pulrerulent, but exhibits the metallic lustre with considerable brilliancy ; has a blue colour brighter than that of zinc; a specific gravity of or 21.4, and, at a temperature higher than that of melted zinc, is combustible. Heated to bright redness, with eight times its weight of tin, in a carbon crucible, and the melted mass allowed to cool slowly, it separates out in a crystalline form, and is isolated on dissolving out the tin with hydro chloric acid. Heated by the oxyhydrogen blowpipe, in a furnace made
of lime, it does not fuse, but aggregates into a more or less compact mass, and at the heat of melted iridium is slowly dissipated in vapour.
Osmium combines with mercury ; the amalgam is decomposed by heat, the mercury volatilising, and the osmium remaining behind in such a fine state of division as to take fire on coming into contact with air ; in this condition also it is soluble in nitric acid, but is not so after ignition. In classifying the elements, osmium is usually placed next to arsenic ; its equivalent is 99'41.
Oxygen and osmium combine in five proportions, forming :— 1. Protoxide of osmium (080).—When potash is added to a solution of chloride of osmium, the protoxide is precipitated in the state of hydrate (OsO, HO), and is of so dark a green colour as to appear nearly black. This hydrate dissolves slowly in acids, forming dark green coloured solutions, which do not yield crystals by evaporation, but leave green or greenish-brown translucent residues. When the hydrate is heated rather below redness, it loses water. It detonates when heated with certain combustible bodies.
2. Sesquioxide of osmium has not been procured in a sepa rate state, but is obtained in combination with ammonia when the binoxide is heated with excess of that alkali ; nitrogen is evolved during the operation by the decomposition of a portion of the binoxide and of the ammonia.
3. Binoxide of osmium (0800 is formed and precipitated by pouring a solution of carbonate of soda into a saturated solution of bichloride of osmium and potassium. The precipitation takes place gradually, and after separating, by means of hydrochloric acid, a little soda which it retains, it is obtained in a state of purity; it is of a black colour and insoluble in acids.
4. Teroxide of osmium osmious acid, is prepared from the terchloride in the same way as the binoxide from the bichloride. Its existence, however, is not perfectly established. A crystalline com pound with potash 2H0) appears to exist.