984 CYANOGEN. 



looking upon the ferrocyanide of potassium as a double cyanide, 

 the cyanide of iron is converted into carburet of iron and nitro- 

 gen gas, while the cyanide of potassium escapes decomposition. 

 The cyanides of metals in general which can combine with 

 carbon, are decomposed in the same way as the cyanide of iron ; 

 thus the cyanide of silver when heated gives at first a little 

 cyanogen, but afterwards it fuses, and glowing suddenly, gives 

 nitrogen gas, the carbon remaining in combination with the 

 silver. The addition of carbonate of potash to the heated ferro- 

 cyanide of potassium prevents the decomposition of any cyano- 

 gen, cyanide of potassium being then formed, together with 

 oxide of iron ; and when charcoal forms a third ingredient of 

 the fused mixture, the oxide of iron is reduced to the metallic 

 state. Hence ferrocyanide of potassium cannot be supposed 

 ready formed in the red-hot mixture of the iron pot in which it 

 is manufactured, that mixture containing both charcoal and car- 

 bonate of potash. 



In the manufacture of this salt, animal substances, such as 

 dried blood, horn, hoofs, and bristles, with common pearlashes, 

 are the materials employed. The animal matter is used either 

 in its natural state, or it is previously submitted to distillation, 

 as in the preparation of ammonia, and the residual charcoal 

 merely employed for the manufacture of the prussiate. The 

 projection of animal matter into the melted potash occasions a 

 lively effervescence, from the evolution of carbonic acid and 

 some combustible gases. The liquid is stirred after each addi- 

 tion of the materials. The usual proportions employed are 

 equal parts of pearlashes and animal matter, or ten parts of the 

 former and eight parts of carbonised animal matter. Three or 

 four per cent of iron filings are usually added to the mixture. 

 After each addition of animal matter the heat is urged until the 

 whole is fused, and the melted material, which is of a thick con- 

 sistence, is not removed from the pot until the charcoal is seen 

 to be equally diffused through the whole mass. The mass, after 

 cooling, is placed in an iron pan filled with water, the clear 

 liquid after a time drawn off, and water boiled several times on 

 the insoluble residue. The liquids are evaporated for crys- 

 tallizing the salt at a temperature not exceeding 203 Fahr. 

 The formation of prussiate takes place after the solution of the 



