This valuable metal occurs pretty abundantly in platinum and osmium alloys, forming 27 to 77 Per cent. of the mineral platiniridium, and 53 1/2 to 58 Per cent. of osmiridium. The preparation of pure iridium has been made a specialty by Johnson and Matthey, the well - known refiners and assayers, and is thus described by 6. Matthey, F.R.S.:-

The metal is prepared with the object of being used in iridio - platinum alloy, consequently it is not necessary to free it from platinum, and the process is devoted only to the removal of all other impurities. In practice, the purest iridium which can be obtained from its ordinary solution (deprived of osmium by long boiling in aqua - regia, and precipitated by ammonium chloride), will almost invariably contain traces of platinum, rhodium, ruthenium, and iron. Such iridium, in a fine state of division, is fused with 10 times its weight of lead, keeping it in a molten state for some hours; the lead is dissolved out by nitric acid, and the residue is subjected to prolonged digestion in aqua - regia, yielding a crystalline mass composed of iridium, rhodium, ruthenium, and iron, in a condition suitable for further treatment. By fusion at a high temperature with an admixture of potash bisulphate, the rhodium is almost entirely removed, any remaining trace being taken up together with the iron in a later operation.

The iridium, so far prepared, is melted, with 10 times its weight of dry caustic potash and 3 times its weight of nitre, in a gold pan or crucible, the process being prolonged for a considerable time to effect the complete transformation of the material into potash iridiate and ruthenate, as well as the oxidation of the iron; when cold, the mixture is treated with cold distilled water. The potash iridiate, of a blue tinge, will remain as a deposit almost insoluble in water, more especially if slightly alkaline; the iron oxide also remains. The precipitate is well washed with water charged with a little soda hypochlorite and potash till the washings are no longer coloured; then several times with distilled water. The blue powder is mixed with water strongly charged with soda hypochlorite, and allowed to remain for a time cold; then warmed in a distilling vessel, and finally brought to boiling - point until the distillate ceases to give a red colour to weak alcohol acidulated with hydrochloric acid. The residue is again heated with nitre and potash - water charged with soda hypochlorite and chlorine, until the last trace of ruthenium has disappeared. The blue powder (iridium oxide) is re - dissolved in aqua - regia, evaporated to dryness, re - dissolved in water, and filtered.

The dark-coloured solution thus obtained is slowly poured into a concentrated solution of soda, and mixed with soda hypochlorite; it should remain as a clear solution without any perceptible precipitate, and, when subjected in a distilling apparatus to a stream of chlorine gas, should not show a trace of ruthenium when hydrochloric acid and alcohol are introduced into the receiver. In this operation the chlorine precipitates the greater part of the iridium in a state of blue oxide, which, after being collected, washed, and dried, is placed in a porcelain or glass tube, and subjected to the combined action of carbon oxide and carbonic acid, obtained by gently heating a mixture of oxalic and sulphuric acids. The iridium oxide is reduced by the action of the gas, leaving the iron oxide intact; the mass is then heated to redness with potash bisulphate (which will take up the iron and any remaining trace of rhodium), and after subjecting it to many washings with distilled water, the residue is washed with chlorine water to remove any trace of gold, and finally with hydrofluoric acid in order to take out any silica which might have been accidentally introduced with the alkalies employed, or have come off the vessels used.

The iridium, after calcination at a strong heat in a charcoal crucible, is melted into an ingot; and after being broken up and boiled in hydrochloric acid, to remove any possible trace of iron adhering to it through the abrasion in breaking up, should possess, if perfectly pure, a density of 22.39; but as iridium, prepared even with the utmost care, will still contain minute though almost inappreciable traces of oxygen, ruthenium, rhodium, and possibly iron,'the highest density attained by Matthey was 22.38.

Deville et Debray's method of purifying unrefined iridium is as follows: - The metal is ignited with barium nitrate, and the mass is treated with water, when the residue consists of iridium oxide and barium osmate; this is boiled with nitric acid to remove the osmium, which volatilizes as tetroxide; the iridium oxide is precipitated from the residual solution by baryta, dissolved in aqua - regia, and then thrown down as iridio - ammonium chloride; on ignition, this yields spongy iridium, containing small quantities of platinum, ruthenium, and rhodium; ignited with potassium nitrate and treated with water, potassium ru - thenate is dissolved; the residue, fused with lead, gives a regulus, which, treated with nitric acid and aqua - regia, affords the pure metal.

Compact iridium is oxidized when heated with acid potassium sulphate, or in presence of fused alkalies; when pure, it is not attacked by aqua - regia, but dissolves when alloyed with much platinum; it is slightly volatile, but extremely difficult of fusion; it has a sp. gr. of 22.38, and a colour resembling polished steel; it is very brittle when cold, but somewhat malleable at a white heat; when an alcoholic solution of the sulphate is exposed to sunlight, the metal is deposited as an impalpable powder, and the merest trace of this iridium - black, washed with hot water, dried, and thrown on paper saturated with alcohol, produces ignition, the metal being converted into grey sponge. The only important use to which the metal has been devoted is for the preparation of an alloy suitable for standard measures; this alloy, consisting of 1 part pure iridium and 9 platinum, is extremely hard, as elastic as steel, less fusible than platinum, perfectly unalterable in the air, and capable of taking a fine polish; some standard rules made by Johnson & Matthey gave the following composition: 89.41 platinum, 10.17 iridium, 0.17 rhodium, 0.10 rubidium, 0.06 iron.