This metal is met with chiefly in the natural alloy osmiri-dium, in proportions varying from 27 1/4 to 43 1/2 per cent., as well as in minor quantities in platiniferous minerals. It is obtained in the form of a more or less pure and very volatile tetroxide in the course of preparing the other members of the platinum group by the following process of Deville et Debray:- The os-miridium ore is fused with zinc, the regulus is treated with hydrochloric acid, the finely divided residue is mixed with the same quantity of barium nitrate and 3 times the quantity of barium binoxide, and the mass is heated for 2 hours at about 1800° F. (982° C.); the resulting compound is cooled, ground very fine, and put into a stoppered vessel containing dilute hydrochloric acid, which must be kept cool and placed in a thorough draught, to prevent the poisonous osmium tetroxide vapours escaping. On completion of the reaction, 2 parts sulphuric and 1 of nitric acid are added, the mixture is well shaken, the barium sulphate is allowed to deposit, and the clear liquid is poured off; the residue is washed by decanta-tion, and f of the liquid are distilled over; this distillate is precipitated by ammonia and ammonium sulphide, and the precipitate is mixed with sodium chloride, and heated in a slow current of chlorine; the lixiviated mass gives sodium osmichloride solution, from which sal-ammoniac precipitates ammonium osmichloride, and this, washed with sal-ammoniac solution and heated in a covered crucible, yields spongy osmium.
Deville et Debray also obtain pure osmium in the form of an amorphous powder by passing the vapour of the pure tetroxide, mixed with carbon monoxide and dioxide, through a red-hot porcelain tube. Crystalline osmium may be produced by fusing the powder with 3 or 4 times its weight of tin in a charcoal crucible, treating the alloy with acid, and heating the residue in a current of hydrochloric acid gas. Metallic osmium has a sp. gr. of 22.477; it is infusible, but evaporates at the temperature reached when iridium is fully melted, and deposits as a black powder on cold surfaces introduced into the vapour. Its alloy, osmiridium, is used for tipping gold pens and for compass bearings, being inoxidizable, proof against acids, and not magnetic.