CYAN1DK, OF POTASSIUM. 451 



through a solution of the ferrocyanide of potassium, till it no 

 longer gives a precipitate of prussian blue with a persalt of iron. 

 One fourth of the potassium of the ferrocyanide is converted into 

 chloride, from which the resulting ferricyanide may be sepa- 

 rated by crystallization. It forms right rhombic prisms, which 

 are transparent and of a fine red colour. The crystals are an- 

 hydrous, soluble in 3.8 parts of cold, and in less hot water. 

 They burn with brilliant scintillations when held in the flame of 

 a candle. The solution of this salt is a delicate test of iron in 

 the state of protoxide, throwing down from its salts a variety of 

 prussian blue, in which the 3K of the formula are replaced by 

 3 Fe. Liebig views this salt as containing a salt-radical, Ferricy- 

 anogen or ferridcyanogen, Fe 2 Cy 6 , differing from ferrocyanogen 

 in having twice its atomic weight and being tribasic. 



Cyanide of potassium ; KCy, 819.8 or 65.69. The prepa- 

 ration of this salt is attended with difficulty, owing to the action 

 of the carbonic acid of the air upon its solution, which evolves 

 hydrocyanic acid, and the tendency of the solution itself to un- 

 dergo spontaneous decomposition, even in close vessels. It 

 may be formed by adding absolute hydrocyanic acid, or a strong 

 solution of that acid, to a solution of potash in alcohol ; a por- 

 tion of the cyanide falls down as a white crystalline precipitate, 

 which should be washed with alcohol and dried, and an addi- 

 tional quantity is obtained by evaporating the liquid in a retort. 

 But it is prepared with more advantage from the ferrocyanide 

 of potassium already described. That salt is carefully dried 

 and reduced to a fine powder, which is exposed to a strong red 

 heat in a well closed iron crucible, or other convenient vessel, 

 and then allowed to cool completely without exposure to air. 

 The porous, semifused mass, which is a mixture of cyanide of 

 potassium and carburet of iron, is reduced to a fine powder, 

 placed in a funnel, moistened with a little alcohol, and then 

 washed with cold water. The first strong solution of cyanide 

 of potassium which comes through is colourless, and must be 

 rapidly evaporated to dryness in a porcelain bason, and fused at 

 a red heat. Or, alcohol of sp. gr. 0.896 (60 per cent) may be 

 boiled upon the black mass, and dissolves a large quantity of 

 cyanide, the greater proportion of which it deposits again on 

 cooling, a property peculiar to alcohol of the strength pre- 

 scribed. The application of hot water to the black mass is to 

 be avoided, as with access of air, it causes the reproduction of 



