346 'PRINCIPLES OF CHEMISTRY 



derived from ferrous and ferric oxides. Amongst these the series, 

 of cyanogen compounds is particularly interesting j their formation, 

 and character is not only determined by the property which, irort 

 possesses of forming complex types, but also by the similar faculty of 

 the cyanogen compounds, -which, like nitriles (Chapter IX.), have 

 clearly developed properties of polymerisation and in general of forming 

 complex compounds. 27 



In the cyanogen compounds of iron, two degrees might be expected : 

 Fe(CN) 2 , corresponding 'with ferrous oxide, and Fe(CN) 3 , correspond- 

 ing with ferric oxide. There are actually, however, many other known 

 compounds, intermediate and far more complex. They correspond 

 with the double salts so easily formed by metallic cyanides. The two 

 following double salts are particularly well known, very stable, often^ 

 used, and easily prepared. Potassium ferrocyanide or yellow prussiate 

 of potash, a double salt of cyanide of potassium and ferrous, cyanide, 

 has the composition FeC 2 N 2 ,4KCN ; its crystals contain 3 mol. of water : 

 K 4 FeC 6 N 6 ,3H 2 0. The other is potassium ferricyanide or red prussiate 

 of potash. It is also known as Gmelin's salt, and contains cyanide of 

 potassium with ferric cyanide ; its composition is Fe(CN) 3 ,3KCN or 

 K 3 FeC 6 N 6 . Its crystals do not contain water. It is obtained from, 

 the first by the action of chlorine, which removes one atom of the 

 potassium. A whole series of other ferrocyanic -compounds correspond- 

 with these ordinary salts. 



Before treating of the preparation and properties of these two 

 remarkable and very stable salts, it must be observed that with ordi- 

 nary reagents neither of them gives the same double decompositions 

 as the other ferrous and ferric salts, and they both present a series of 

 remarkable properties. Thus these salts have a neutral reaction, are 

 unchanged by air, dilute acids, or water, unlike potassium cyanide and 

 even some of its double salts. When solutions of these salts are treated 

 with caustic alkalis, they do not give a precipitate of ferrous or fecric 

 hydroxides, neither are they precipitated by sodium carbonate. This 

 led the earlier investigators to recognise special independent groupings 

 in them. The yellow prussiate was considered to contain the complex 

 radicle FeC 6 N 6 combined with potassium, namely with K 4 , and K$ 

 was attributed to the red prussiate. This was confirmed by the feic* 

 that whilst in both salts any other metal, even hydrogen, might be 

 substituted for potassium, the iron remained unchangeable, just as 

 nitrogen in cyanogen, ammonium, and nitrates does not enter into 



27 Some light may be thrown upon the faculty of Fe of forming various compounds with 

 CN, by the fact that Fe not only combines with carbon but also with nitrogen. Nitride 

 of iron Fe 2 N was obtained by Fowler by heating finely powdered iron in a stream ol 

 NH 3 at the temperature of jnelting lead.^ 



