5. Peroxide of osmium, or osmic acid (0s0,).—This is a very curious compound, differing remarkably from all the preceding substances. It is obtained whenever osmium is burnt in the air or in oxygen gas, or when the pulverulent metal is heated with hot nitric acid. It is white, very volatile, extremely fusible, soluble in water, and crystallises in colourless transparent flexible needles. It possesses an extremely disagreeable odour, somewhat resembling that of chlorine. It com bines with alkalies, forming compounds which are called osmiates. When sulphurous acid is added to a solution of osmic acid, it is rendered yellow, orange, brown, green, and blue, dependent upon the extent to which the deoxidisement is carried on. It stains the skin, is acid and poisonous.
Osmium and sulphur form four sulphides. They are produced on passing sulphuretted hydrogen through the hydrochloric solutions of the oxides. They much resemble the latter in constitution.
Osmium and chlorine form four chlorides, namely, protochloride of osmium (OsCI), which sublimes in green needles on passing chlorine over osmium heated in a long tube ; scsguichloride of osmium known only in combination with chloride of ammonium; bichloride of osmium formed by excess of chlorine in the preparation of the protochloride, but crystallising farther from the source of heat, and is red instead of green ; and terchloride of osmium also known only in combination with chloride of ammonium.
Osmium and nitrogen form osmiamic acid according to Gerhardt, or osmanosmic acid (08N, 0s0,), according to Fritzsche and Struve. It separates as a yellow crystalline powder on adding caustic 'ammonia to a solution of osmic acid in potash.
The remaining compounds of osmium have not been satisfactorily studied. They are all characterised by giving fumes of osmic acid, when heated with carbonate of soda on platinum foil in the blowpipe flame, and the fumes are recognised by their odour. Osmic acid also imparts luminosity to the flame of alcohol.