386 PRINCIPLES OF CHEMISTRY 



Platinum and its analogues, like iron and its analogues, are able to 

 form complex and comparatively stable cyanogen and ammonia com- 

 pounds, corresponding with the ferrocyanides and the ammoniacal com- 

 pounds of /cobalt, which we have already considered in the preceding 

 chapter. 



If platinous chloride, PtCl a (insoluble in water), be added by degrees 

 to a solution of potassium cyanide, it is completely dissolved (like 

 silver chloride), and on evaporating the solution deposits rhombic 

 prisms of potassium platinocyanide t PtK 2 (CN) 4 ,3H 2 0. This salt, like 

 all those corresponding with it, has a remarkable play of colours, due to 

 the phenomena of dichromism, and even polychromism, natural to all 

 the platinocyanides. Thus it is yellow and reflects a bright blue 

 light. It is easily soluble in water, effloresces in air, then turns red, 

 and at 100 orange, when it loses all its water. The loss of water 

 does not destroy its stabilitythat is, it still remains unchanged, and 

 its stability is further shown by the fact that it is formed whea 

 potassium ferrocyanide, K 4 Fe'(CN) 6 , is heated with platinum black. 

 This salt, first obtained by Gmelin, shows a neutral reaction with 

 litmus ; it is exceedingly stable under the action of air, like potassium 

 ferrocyanide, which it resembles in many respects. Thus the platinum 

 In it cannot be detected by reagents such as sulphuretted hydrogen ; 

 the potassium may be replaced by other metals by the action of their 

 salts, so that it corresponds with a whole series of compounds, R 2 Pt(CN) 4 , 

 and it is stable, although the potassium cyanide and platinous salts, of 

 which it is composed, individually easily undergo change. When 

 treated with oxidising agents it passes, like the ferrocyanide, into a 

 higher form of combination of platinum. If salts of silver be added 

 to its solution, it gives a heavy white precipitate of silver platino- 

 cyanide, PtAg 2 (CN) 4 , which when suspended in water and treated 

 with sulphuretted hydrogen, enters into double decomposition with the 

 latter and forms insoluble silver sulphide, Ag 2 S, and soluble hydro- 

 platinocyanic acid, H 2 Pt(CN) 4 . If potassium platinocyanide be mixed 

 with an equivalent quantity of sulphuric acid, the hydroplatino- 

 cyanic acid liberated may be extracted by a. mixture, of alcohol and 

 ether. The ethereal solution, when evaporated in a desiccator, deposits 

 bright red crystals of the composition PtH 2 (CN) 4 ,5H 8 0. This acid 

 colours litmus paper, liberates carbonic anhydride from sodium car- 

 bonate, and saturates alkalis, so that it presents an analogy to hydro- 

 ferrocyanic acid. 11 



" This acid character is explained by the influence of the platinum on the hydrogen, 

 and by the attachment of the cyanogen groups. Thus cyanuric acid, H 3 (CN) 3 03, is an 

 energetic acid compared with cyanic acid, HCNO. And the formation of a compound 



