Osmum, a metal belonging to the platinum group, discovered by Tennant in 1803 in platinum ore, associated with iridium, ruthenium, and small quantities of rhodium, as an alloy called osmiridium or iridosmene, and which forms the residue left after the treatment of platinum ore by aqua regia. The method for separating these different metals is that of Fremy, and depends upon the readiness with which osmium is oxidized and upon the volatility of the tetroxide produced. About 200 grammes of the platinum residue is roasted in a current of dry air in a porcelain or platinum tube heated to redness. Tetroxide of osmium or osmic acid is formed, and being volatile is passed into a series of glass flasks connected with the tube leading from the furnace, where it is condensed in beautiful needle-shaped crystals. The last flask contains a solution of caustic potash to absorb any tetroxide that may remain uncondensed, and an aspirator is attached to it to draw the air through the apparatus. The oxide is then by Berzelius's method digested with hydrochloric acid and mercury in a closed vessel at 284° F. Calomel is produced by the decomposition of mercurous oxide which is formed by the union of the mercury with the oxygen of the osmic acid, and the osmium is left in a metallic state in the form of a black powder (Os04 + 8Hg + 8Hcl =Os + 4Hg2Cl2 + 4H20). The metal may also be obtained by digesting osmic acid with hydrochloric acid and zinc.

The properties of osmium vary with the mode of preparation. In the black pulverulent state its specific gravity is about 10, but when heated to the fusing point of rhodium it acquires a density of 21.4. At a still higher temperature, capable of melting ruthenium and iridium and volatilizing platinum, osmium likewise volatilizes, but does not melt; and it is in fact the most refractory of all metals. In a finely divided state it is highly combustible, and is easily oxidized by nitric or nitro-muriatic acid, in both cases being converted into tetroxide. Five oxides of osmium are known, viz., OsO, Os203, Os02, Os03, and Os04. The first three form salts with acids; the fourth forms with a few bases salts called osmites. The tetroxide, often called osmic acid, forms salts which are very unstable; it can scarcely be regarded as a true acid, and its solution in water has no acid reaction with test paper. It is a powerful oxidizing agent, decolorizing indigo solution, and converting alcohol into aldehyde and acetic acid. Its vapor is intensely pungent, irritates the mucous membrane, and is exceedingly poisonous. According to Fremy, another oxide exists, Os05, but it is very unstable.

There are four chlorides, OsCl2, OsCL, OsCl4, and OsCl6, called respectively dichloride, trichloride, tetrachloride, and hexachloride. Osmium burns in the vapor of sulphur, forming it is said fixed sulphides, analogous to the oxides. All osmium compounds when heated with excess of nitric acid yield an unpleasant odor of tetroxide of osmium.