(oMI'ol'NDS OF CARBON WITH OXYt.KN AND NITROGEN 401 



alkaline reaction. 43 If ammonia be passed over charcoal at a red heat, 

 especially in the presence of an alkali, or if gaseous nitrogen be passed 

 through a mixture of charcoal and an alkali (especially potash, KHO), 

 and also if a mixture of nitrogenous organic substances and alkali be 

 heated to a red heat, in all these cases the alkali metal combines with the 

 carbon and nitrogen, forming a metallic cyanide, MCN for example, 

 KCN. Potassium cyanide is much used in the arts, and is obtained, 

 as above stated, under many circumstances as, for instance, in iron 

 smelting, especially with the assistance of wood charcoal, the ash of 

 which contains much potash. The nitrogen of the air, the alkali of the 

 ash, and the charcoal are brought into contact at a high temperature 

 during iron smelting, and therefore, under these conditions, a consider- 

 able quantity of potassium cyanide is formed. In practice it is not usual 

 to prepare potassium cyanide directly, but a peculiar compound of it 

 containing potassium, iron, and cyanogen. This compound is potassium 

 ferrocyanide, and is also known as yellow prussiate of potash, because 

 with ferric salts it forms a blue colour Prussian blue. This saline 

 substance, having the composition K 4 FeC 6 N 6 + 3H 2 O, is prepared on 

 the large scale in cast-iron pots (the iron of which enters into the salt), 

 by heating with alkali leather cuttings, horn filings and shavings, 

 and similar animal matter, composed of complex organic substances 

 containing carbon and nitrogen ; the alkali which is added is com- 

 posed of potashes containing potassium ; thus all the necessary ele- 

 ments for producing the yellow prussiate are brought into contact at a 

 high temperature, and the above-mentioned substance is formed. 

 The name of cyanogen (/cvai/o?) is derived from the property which 

 the yellow prussiate possesses of forming, with a solution of a ferric 

 salt, FeX 3 , the familiar pigment Prussian blue. The yellow prussiate 

 is generally used as the source of the other cyanogen compounds 

 (because it is manufactured on a large scale), and we will therefore 

 describe the method of forming the cyanogen compounds derived 

 from it. 



If yellow prussiate be mixed with two parts of water and three- 



Solutions of cyanides for instance, of those of potassium or barium are decom- 

 carbonic acid. Even the carbonic anhydride of the air acts in a similar way, 

 and for this reason these solutions do not keep, because, in the first place, free hydro- 

 cyanic acid itself decomposes and polymerises, and, in the second place, with alkaline 

 liquids it forms ammonia and formic acid. Hydrocyanic acid does not liberate carbonic 

 anhydride from solutions of sodium or potassium carbonates. But a mixture of solutions 

 of potassium carbonate and hydrocyanic acid yields carbonic anhydride on the addition 

 of oxides similar to zinc oxide, mercuric oxide, &c. This is due to the great inclination 

 which the cyanides evince to form double salts. For instance, ZnK.>(CN)4 is formed, 

 which is a kind of soluble double salt. 



VOL. I. D D 



