OSMIUM, a chemical element found in small quantities in platinum ores but obtained chiefly from the mineral osmiridium by the method of Deville and Debray (see RUTHENIUM) to gether with iridium it was first isolated by Smithson Tennant in 1803 (symbols Os, atomic number 76, atomic weight 190.8). Os miridium is fused with metallic zinc, the excess of zinc is dis solved in hydrochloric acid and the residue ignited with a mixture of barium nitrate and barium peroxide. When cold, the mass is finely powdered and treated with cold dilute hydrochloric acid in a stoppered bottle; this is necessary as osmium is present in the form of volatile tetroxide. After all action has ceased, nitric and sulphuric acids are added, the mixture is well shaken and the barium sulphate allowed to deposit. The liquid is then distilled, when the volatile osmium tetroxide passes over in an almost pure state. The residue from the distillation is concentrated, ammonium chloride is added, and ammonium iridium chloride (together with a little ruthenium) is precipitated. Another method of obtaining osmium from osmiridium is to heat the alloy mixed with common salt in a slow stream of chlorine. The sodium osmichloride, produced is extracted with water; sal ammoniac is added to precipitate sparingly soluble ammonium osmichloride, which on gentle ignition leaves a residue of spongy osmium.
Osmium in the massive state is a metal of a bluish-white colour. It can be obtained crystalline by alloying any of its forms with metallic tin and subsequently removing the tin by means of hydrochloric acid. It has a specific gravity of 22.48, and melts at about 2,700° C, although it begins to volatilize at a lower tem perature. It is a very hard metal, especially when combined with iridium, and the alloy was at one time used extensively for tip ping gold pens, but for this purpose cheaper alloys are now em ployed free from platinum metals and generally having tungsten for their basis. It is not suitable in alloys with other platinum metals for high-temperature work, for, owing to the ease with which it oxidizes, it causes the alloy to disintegrate.
Osmium forms several oxides of which four are known with certainty, namely, OsO, and 0s0„ while is pres ent in the metallic osmates. The lower oxides are unimportant. Osmium tetroxide, 0s0„ known in commerce under the name of osmic acid, can readily be obtained in a state of purity, and is the source from which other osmium compounds are generally prepared. If metallic osmium in any form is heated in a tube in a current of air, or preferably oxygen, to a bright red heat, com bination takes place and a yellowish liquid, Osa„ condenses in the cold part of the tube, afterwards crystallizing in yellowish needles which melt at 40° C; the liquid boils at roo° C giving a colourless vapour. Traces of the vapour of osmium tetroxide have a very strong odour somewhat resembling that of chlorine. The vapour is poisonous and exceedingly corrosive and attacks animal membranes vigorously, becoming reduced and leaving a deposit of metallic osmium. In small quantities the physiological effect is transitory but larger quantities entail serious conse quences. It is largely used for preparing microscopic slides as it acts preferentially on certain portions of tissue, staining them black. If alcohol is added to 0s0, dissolved in caustic potash, the solution becomes red and on concentration potassium osmate, crystallizes out. This salt is stable in neutral solutions but decomposes on acidification, forming 050, and lower oxides. Osmium has several chlorides such as and OsCL ; these, and especially the last two, have a great tendency to combine with alkaline chlorides producing double chlorides of • the form and They are readily formed from the tetroxide by the use of suitable reducing agents. Two sulphides of osmium are known, and OsS,; the latter is formed by the action of sulphuretted hydrogen upon acid solu tions of the tetroxide. Qualitatively, osmium is detected by the odour of 050, produced when the metal is ignited in air. A deli cate test for 050, is to warm an aqueous solution with thio carbamide and dilute hydrochloric acid, when a red coloration shows the presence of osmium. (F. E. M.)