DOUBLE CYANIDES. 991 



blue (page 557). The same alkaline ferricyariide does not dis- 

 turb a persalt of iron. 



Sesquicyanide of cobalt, Co 2 Cy 3 , forms with other cyanides a 

 class of double salts strictly analogous to the immediately 

 preceding class, and which are represented as compounds of 

 a tribasic salt-radical, cobalticyanogen, Co 2 Cy 6 , or as cobalti- 

 cyanides. Cobalticyanide of potassium, K 3 ,Co 2 Cy 6 was dis- 

 covered By L. GmeUn, and is prepared by heating slightly a 

 mixture of protoxide of cobalt, or its carbonate, with a solution 

 of potash supersaturated with hydrocyanic acid ; hydrogen is 

 disengaged with a slight effervescence, and the solution when 

 evaporated, furnishes the salt in question in reddish-yellow 

 crystals, which require to be purified by a second crystalliza- 

 tion. It is isomorphous with ferricyanide of potassium. 



Sesquicyanide of manganese, Mn 2 Cy 3 , appears also to form 

 with other cyanides a similar class of salts (page 534). 



Sesquicyanide of chromium, Cr 2 Cy 3 , appears to exist in a 

 class of double cyanides of the same type. 



Chromocyanide of potassium, K 3 ,Cr 2 Cy 6 , is formed when a 

 mixture of hydrated oxide of chromium with a solution of hy- 

 drate of potash to which an excess of hydrocyanic acid has 

 been added, is allowed to evaporate spontaneously in air. The 

 liquid acquires a reddish- brown colour, and deposits yellow crys- 

 tals, which have a similar form and composition with ferri- 

 cyanide or cobalticyanide of potassium. (Boeckmann). 



Bicyanide of platinum, PtCy 2 , is considered by Liebig as 

 existing, as a radical, in a series of platinum compounds, dis- 

 covered by M. Doebereiner. This salt-radical is monobasic. 

 Platinocyanide of potassium, K,PtCy 2 + 3HO, is formed on ex- 

 posing to a heat approaching redness a mixture of equal parts of 

 platinum sponge and dried ferrocyanide of potassium. A solu- 

 tion of the heated mass affords first, when evaporated, crystals 

 of ferrocyanide of potassium, and then of platinocyanide of 

 potassium. The latter crystallizes in thin elongated rhomboidal 

 prisms, which are blue by reflected and yellow by transmitted 

 light. Its solution does not precipitate nitrate of lead, but 

 nearly all the other metallic salts. Platinocyanic acid or pla- 

 tinocyanide of hydrogen, H,PtCy 2 is obtained by decomposing 

 the platinocyanide of mercury suspended in water by a stream 

 of sulphuretted hydrogen gas. It crystallizes in a confused 



