CYANIDES, DOUBLE CYANIDES. 989 



to allow of the formation of some peroxide of iron, is made 

 acid by the addition of hydrochloric acid, when Prussian blue is 

 formed and precipitates. 



Hydrocyanic acid does not decompose carbonates, and may 

 therefore be purified from other acids by distilling it from chalk. 

 But the acid so distilled cannot be preserved without adding a 

 trace of a mineral acid to it. 



Cyanides. Hydrocyanic acid forms a metallic cyanide and 

 water with red oxide of mercury, oxide of silver and other me- 

 tallic oxides of which the metal has a feeble affinity for oxygen. 

 The cyanides of these metals are not decomposed by dilute 

 oxygen-acids, and resist for a long time the action of concen- 

 trated and boiling nitric acid. But the same cyanides are 

 decomposed very easily by sulphuretted hydrogen and hydro- 

 chloric acid. The cyanide of mercury is resolved when heated 

 into metal and cyanogen, a portion of the latter remaining in 

 the retort as paracyanogen, a black coaly matter, isomeric with 

 cyanogen. Caustic potash is not neutralised by hydrocyanic 

 acid, but remains strongly alkaline, while the solution retains 

 the odour of the acid. Hence it is doubted whether cyanide of 

 potassium is then formed, and rather supposed that hydrocyanic 

 acid unites directly with oxide of potassium. But the solid 

 cyanide of potassium, when dissolved in water, has the same 

 characters as the preceding solution. Cyanide of potassium 

 dissolves the metallic cyanides insoluble in water, forming 

 double salts, and then acquires stability. Cyanide of potassium 

 is decomposed when heated with a solution of caustic potash ; 

 the cyanogen of the former salt, uniting with the elements of 

 water, is converted into formic acid and ammonia : 



K,NC 2 and 4HO=KO + C 2 HO 3 and NH 



3 . 



The insoluble cyanides of all the non-alkaline metals may be 

 obtained by adding hydrocyanic acid to an acetate of the 

 metal. 



DOUBLE CYANIDES. 



The soluble cyanides of potassium and sodium dissolve all the 

 insoluble cyanides of the metals proper, and form double com- 



