PRUSSIAN BLUE. 331 



formation of ferrocyanide, gives off a considerable proportion 

 of heat: viz. 39'3 - 8*7 = + 30'6. Moreover, only a single 

 equivalent of ferrous oxide is required to constitute hydroferro- 

 cyanic acid. 



These figures also explain the displacement observed, and 

 correspond to the constitution of a new molecular type, that 

 of hydroferrocyanic acid. 



In fact, we conclude from them that 



i[6HCN (in solution) + FeO (precipitated)] gives off + 12*3, 



which quantity exceeds the heat (-f 9'0) disengaged by the 

 combination of i[3K 2 (diluted)] with i[6HCN]. 



There are here two simultaneous reactions, viz. the union 

 of six molecules of hydrocyanic acid into a type six times as 

 condensed, and the reaction of the ferrous oxide which enters 

 into the constitution of this new type : H 4 Fe(GN") 6 . 



In the same way, for Prussian blue, the fact has been 

 established elsewhere that 



i[3H 4 Fe(CN) 6 (diluted) + 2Fe 2 3 (precipitated)] gives off + 12-6 



= 6-3 x 2, 



or almost the same quantity as in the union of the same oxide 

 with diluted hydrochloric and nitric acids. 

 From hydrocyanic acid itself we get 



i[18HON (diluted) + 3FeO + 2Fe 2 3 = Fe 7 (CN) l8 (precipi- 

 tated)] + 24-9 = 8-3 x 3. 



The magnitude of this last quantity, which is three times that 

 of the heat disengaged by potash in its combination with hydro- 

 cyanic acid, enables us, as before, to account for the formation 

 of the new molecular type of ferrocyanides, and, in a more 

 general manner, for the formation of double cyanides. 



11. This consistency in effects also explains, by the greater 

 quantity of heat disengaged, the greater apparent affinity pre- 

 sented by ferrous oxide, as compared with potash, in its union 

 with hydrocyanic acid ; this is contrary to what happens in the 

 comparison of the formation of the ordinary oxysalts, sulphates, 

 nitrates, acetates, etc., from diluted acids and alkaline bases, 

 with that of those salts from the same salts with metallic 

 oxides. 



12. Could we not explain by some similar circumstance why 

 silver and mercuric oxides, as well as ferrous oxide, give off 

 more heat than diluted potash in their combination with hydro- 

 cyanic acid? In a word, are mercuric and silver cyanides 

 really represented by the simple formulae 



AgCN and Hg(CN) 2 , 



those of potassium cyanide and hydrocyanic acid being KCN 

 and HCN ? or ought we rather not regard them as being them- 



