882 PRINCIPLES OF CHEMISTRY 



or when acted upon by substances capable of taking up chlorine (even 

 alkalis, which form bleaching salts). Among the platinum metals, 

 ruthenium and osmium have the most acid character, and although they 

 give RuCl 4 and OsCl 4 they are easily oxidised to Ru0 4 and Os0 4 by 

 the action of chlorine in ibhe presence of water ; the latter are "volatile 

 and may be distilled with the water and hydrochloric acid, from a 

 solution containing other platinum metals. 9 trl Thus with respect to 



the black hydroxide, like many other oxidised compounds of the platinoid metals, does 

 not dissolve in the ordinary- oxygen acids, whilst the yellow hydroxide is easily soluble 

 and gives yellow solutions, which deposit imperfectly crystallised salts. Metallic, 

 rhodium ifl easily obtained by igniting its oxygen and other compounds in hydrogen, or 

 by precipitation with zinc. It resembles platinum, and has a sp. gr. of 12-1. At the 

 ordinary temperature it decomposes formic acid into hydrogen and carbonic anhydride, 

 with development of heat (Deville). With the alkali sulphites, the salts of rhodium and 

 iridiuni of the type #X 3 give sparingly-soluble precipitates of double sulphites of the 

 composition R(S0 3 Na) 3 ,H a O, by means of which these metals may be separated from 

 solution, and also may be separated from each other, for a mixture of these salts when 

 treated with strong sulphuric acid gives a soluble iridium sulphate and leaves a red in- 

 soluble double salt of rhodium and sodium. It may be remarked that the oxides Ir a O3 and 

 Eh 2 O 3 are comparatively stable and are easily formed, and that they also form different 

 aouble salts (for instance, IrCl 3 ,3KCl3H 2 O, RhCl 3) 2NH 4 Cl 4 H 3 0, RhCl^SNHiClliH-jO) 

 and compounds like the cobaltia compounds (for instance, luteo-salts RhX 5 ,6NH 3 , roseo- 

 salts, RhX 3 H 2 O5NH 3 , and purpureo-salts IrX 3 ,5NH 3 . &c.) Iridious oxide, Ir 2 O 3 , ia 

 obtained by fusing iridious chloride and its compounds with sodium carbonate, and 

 treating the mass with water. The oxide is then left as a black powder, which, when 

 strongly heated, is decomposed into iridium and oxygen; it is easily reduced, and ia 

 insoluble in acids, which indicates the feeble basic character of this oxide,, in many 

 respects resembling such oxides as cobaltic oxide, cerio or lead dioxide, &o. It does not 

 dissolve when fused with potassium hydrogen sulphate. Rhodium oxide, Rh-jOs, is a 

 tar more energetic base. It dissolves when fused with potassium hydrogen sulphate. 



From what has been said respecting the separation of platinum and rhodium it will 

 be understood how the compounds of iridium, which is the main associate of platinum, 

 are obtained. Ia describing the treatment of osmiridium we shall again have an 

 opportunity of learning the method of extraction of the compounds of this metal, which 

 has in recent times found a technical application in the form of its oxide, Ir 2 O 3 & 

 this is obtained from many of the compounds of iridium by ignition with water, is 

 easily reduced by hydrogen, and is insoluble in acids. It is used in painting on china, 

 for giving a black colour. Iridium itself is more difficultly fusible than platinum, and 

 when fused it does not decompose acids or even aqua regia ; it is extremely hard, and is 

 not malleable ; its sp. gr, is 22*4. In the form of powder it dissolves in aqua regia, and 

 is even partially oxidised when heated in air, sets fire to hydrogen, and, in a word, closely 

 resembles platinum. Heated in an excess of chlorine it gives iridio chloride. IrCl^ but 

 this loses chlorine at 50 ; it Is, however, more stable in the form of double salts, which 

 have a characteristic black colour for instance, Ir(NH4) 2 Cl 6 but they give iridious 

 chloride, IrCl 3 , when treated with sulphuric aeid. 



9 tr| We have yet to become acquainted with the two remaining associates of platinum 

 ruthenium and osmiumwhose most important property is that they are oxidised 

 even when heated in air, and that they are able to give volatile oxides of the form Ru0 4 

 and Os0 4 ; these have a powerful odour (like iodine and nitrous anhydride). Both these 

 higher oxides are solids ; they volatilise with great ease at 100 ; the former is yellow 

 and the latter white. They are known as ruthenic and osmic anhydrides, although their 

 aqueous solutions (they both slowly dissolve in water) do not show an acid reaction, and 

 although they do not even expel carbonic anhydride from potassium carbonate, do not 



