Osmium (Os, at.wt. 190-3), a rare metal which in its properties is closely allied to platinum, belonging to the platinum group of metals. Its chief source is the ore known as osmiridium. consisting chiefly of a compound with iridium. This ore is so hard and unalterable that it is employed for bearings where the most extreme hardness is desirable. It is for the same reason employed for tipping stylographic pens. It occurs chiefly in California, the Urals, and at Katherinenberg. Its specific gravity is the highest of all known substances, being 2243. Its salts have not been much studied, and are only of chemical interest; but one of its oxides, osmic acid, is used in dilute solution for hardening animal tissues for microscopic preparations. At 100° C. it gives off a very offensive odour, hence the name (Gk. osme, smell).