412 PRINCIPLES OF CHEMISTRY 



hydrocyanic acid does not give an acid reaction with litmus (cyanic 

 acid' has very marked acid properties) ; but it forms salts, MCN y 

 thus presenting the properties of a feeble acid, and for this reason is 

 called an acid. The small amount of energy which it has is shown 

 by the fact that the cyanides of the alkali metals" for instance, potas- 

 sium' cyanide (KHO + HCN == H ? O + KCN) in solution have a 

 strongly alkaline reaction. 43 If ammonia be passed over charcoal at 

 a red hea,t, '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. 43bis ' Potassium cyanide is much 

 used in the arts, and is obtained, as above stated, under many circum- 

 stances as, for instance, in iron smelting, especially with the assistance 

 of wood charcoal, the ash of wh'ich 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, 

 tinder these conditions, a considerable 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. This saline substance (see 

 Chapter XXII) has the composition K 4 FeC 6 N 6 + 2H 2 O. The name 

 of cyanogen (icuai/o?) is derived from the property which this yellow 

 prussiate possesses of forming, with a solution of a ferric salt, FeX 3 , 

 the familiar pigment Prussian blue. The yellow prussiate is manu- 



45 Solutions of cyanide* for-'instance, those of potassium, or "barium are decom- 

 posed by 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 Jiydro- 

 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 like zinc oxide, mercuric oxide, &c. This is due to the great inclination which 

 the cyanides exhibit of forming double salts. For instance, ZnK 2 (CN) 4 is formed, which 

 is a soluble double salt. 



45 bu TTheqpnyersibijof the atmospheric nitrogen into cyanogen compounds, although 

 possible, has not yet been carried out on a large scale, and one of the problems for future 

 research should be the discovery of a practical .and economical means of converting the 

 atmospheric nitrogen into metallic cyanides, not only because ~ potassium cyani.de has 

 found a vast and important use for the extraction of gold from .even the poorest ores, but 

 more especially because the cyanides furnish the means for effecting the synthesis of 

 'many complex carbon compounds, and the nitrogen contained in cyanogen easily passes 

 into other forms of combination such as ammonia, which is of great importance in 

 agriculture. 



