332 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



selves cyanides of a more condensed type, such as Hg 2 (CN) 4 

 and Ag 2 (CN) 2 ? The heat disengaged by their union with 

 potassium cyanide, so as to form double cyanides, such as 

 K 2 Hg(CN) 4 ; KAg(CN) 2 , even in dilute solutions, would support 

 this opinion ; as it would result from the transition from the 

 simple type potassium cyanide to the complex type constituting 

 the double cyanides 



Hg(CN) 2 + 2KCN = HgK 2 (ON) 4 



AgCN + KCN = AgK (CST) 2 . 



Besides, hydrocyanic acid is not the only acid that gives 

 rise to the general inversion of ordinary affinities, as shown by 

 the corresponding thermal effects, between the alkaline oxides 

 and metallic oxides. It is exactly the same with hydrosul- 

 phuric acid. 1 



13. However it may be with regard to these last consider- 

 ations, it remains no less a fact that metallic oxides give off 

 more heat than alkaline bases in uniting with hydrocyanic 

 acid; which explains why they displace them. It may be 

 repeated that thermo-chemistry thus explains the constitution 

 of complex cyanides new molecular types, very superior to the 

 primitive type, as regards the energy of their affinities towards 

 bases, and the stability of the resultant salts, very superior to 

 hydrocyanic acid, which aids in their formation by its conden- 

 sation. 



Hydrocyanic acid, the common generator of these condensed 

 types, is moreover distinguished by the fact that it is formed 

 from the elements with an absorption of heat amounting to 

 29-5 ; in other words, its formation has stored up an excess of 

 energy that gives it a special tendency towards successive 

 combinations and molecular condensations. 



14. We will give, in conclusion, the heat of formation of 

 potassium ferrocyanide from its elements, a quantity that 

 enters into the study of certain explosive substances. 



From cyanogen, we should get 



J[2Ka + Fe + 3(CN) 2 = K 4 Fe(CN) 6 (solid)] gives off + 183*6, 



or + 61-2 x 3. 

 From the elements 

 [2K 2 + Fe + 60 + 31ST 2 = K 4 Fe(C]Sr) 6 ] + 717, or + 23'9 x 3. 



These values are near those that correspond to the formation 

 of potassium cyanide ; viz. from cyanogen, + 67'6 ; and from 

 the elements, + 30'3. 



The hydrated salt contains, in addition, three equivalents of 

 water, 3H 2 0, the union of which in a liquid form with the 

 anhydrous salt gave off 2[ -j- 2 *48] ; which makes altogether for 

 the crystallised yellow prussiate from the elements and water, 

 + 94-2 Cal.] 



1 See " Annales de Chimie et de Physique," 5 e se'rie, torn. iv. p. 186. 



