POTASSIUM FERROCYAXIDE. 327 



off only + 1'7 Cal., instead of -f 4*8, which would correspond 

 to a total decomposition. Tartaric acid gives similar results. 

 It would seem, then, that the liquors contain a hydro-argento- 

 cyanic acid, already mentioned by Meillet ; a complex acid, 

 which can only exist in the presence of water and another acid, 

 so as not to give rise to phenomena of equilibrium, and con- 

 sequently to a partial decomposition. The solutions of this 

 complex acid produce results in silver-plating that are almost 

 as well marked as those produced by alkaline cyanide solutions, 

 as there was occasion to prove. 



This compound forms a very remarkable intermediate step in 

 the formation of those special molecular types that constitute 

 the complex cyanides. 



3. Potassium ferrocyanide. A more decided stability charac- 

 terises the double cyanide of potassium and iron, known as 

 ferrocyanide. Although the thermal study of its formation 

 presents great difficulties, owing to the fact that we cannot start 

 with isolated iron cyanides, nevertheless it is undoubtedly worth 

 while giving the results of the experiments performed, with the 

 admission that they are, no doubt, imperfect. 



4. The heat of solution of both dry and hydrated potassium 

 ferrocyanide was first measured, the former in fifty parts of 

 water, the latter in forty parts of water. It was found that 

 at 11 



^[K 4 Fe(CN) 6 , 3H 2 0] (211-2 grins.), in dissolving, absorbs - 846. 

 i[K 4 Fe(CN) 6 (dry)J - 5-98. 



From these figures it follows that the union of the water with 

 the dry salt 



i[K 4 Fe(CN) 6 + 3H 2 (solid) = K 4 Fe(CN) 6 , 3H 2 (crystal)], gives 

 off + 0-34, or + Oil for each 



a quantity which is very small, but, according to certain experi- 

 ments, 1 comparable to that which is disengaged in the formation 

 of the hydrated calcium and copper acetates. 



5. The heat of neutralisation of hydroferrocyanic acid by bases 

 cannot be conveniently measured directly, on account of the 

 difficulty of obtaining the free acid in a perfect state of purity. 

 The latter object was attempted by indirect means, i.e. by dis- 

 placing the acid from its salts by more powerful acids. 



On mixing a diluted solution of ferrocyanide 



i[K 4 Fe(CN) 6 ] = 4 litres, 



with diluted hydrochloric acid (1 eq. = 2 litres), we observe 

 that there is absolutely no change of temperature, either because 

 there is no reaction, or because the two acids disengage the 

 same quantity of heat in acting on the potash, in which case 



1 " Annales de Chimie et de Physique," 5 e se*rie, torn. iv. p. 127. 



