6J)0 IR1DIUM. 



is the state of oxidation which iridium most readily assumes. 

 Berzelius recommends as the best process for procuring it, to 

 mix the double bichloride of iridium and potassium (K C1 + 

 Ir C1 2 ) with twice its weight of carbonate of potash, and to ex- 

 pose it to a low red heat. On dissolving out the alkaline salt, 

 the deutoxide remains as a very fine powder, of a black colour 

 with a shade of blue. A heat above the melting point of silver 

 is required to expel the oxygen from this oxide. It is reduced 

 to the metallic state by hydrogen gas at the usual temperature, 

 which appears to arise from the oxide of iridium having the 

 property, as well as the metal, to determine the oxidation of 

 hydrogen, a reaction which causes the oxide to be heated to the 

 temperature at which it is itself reduced by hydrogen. The 

 hydrate of this oxide dissolves in acids and forms a particular 

 class of salts, of which the solutions are sometimes of a very 

 dark colour, resembling a mixture of water and venous blood. 

 Binoxide of iridium^ or iridic oxide, Ir O 2 , has not been ob- 

 tained in a separate state, but exists in a class of salts, from 

 which this oxide is not precipitated by an alkali. Peroxide of 

 iridium, Ir O 3 , or susiridic oxide, is formed in small quantity 

 when the alloy of osmium and iridium fused in nitre is digested 

 in aqua regia. The double perchloride of iridium and potas- 

 sium then formed yields a rose-red solution. The salts of the 

 protoxide and peroxide afford blue and purple solutions when 

 mixed, depending probably on the formation of one or more 

 combinations of these oxides. The name iridium (from Iris) 

 was applied to this metal, from the variety of colours which its 

 preparations exhibit. 



Sulphurets of iridium corresponding with the oxides of the 

 same metal have been formed. 



Chlorides of iridium. The protochloride, Ir Cl, is formed 

 when iridium in powder is heated to low redness in chlorine gas. 

 As thus prepared it is insoluble in water, but slightly soluble in 

 hydrochloric acid. It forms double salts with chlorides of po- 

 tassium, ammonium and sodium. The sesqui-chloride, Ir 2 Cl 3 , 

 also forms double salts, but which are not crystallizable. The 

 bichloride of iridium forms a double salt with chloride of potas- 

 sium, in brilliant black octohedral crystals, corresponding with 

 the bichloride of platinum and potassium. The bichloride of 

 iridium arid sodium is also isomorphous with the corresponding 

 platinum salt. 



