DOUBLE CYANIDES. 325 



The same differences exist for cyanides when compared with 

 bromides (Br gas) 



Difference. 



K] 85-4 - 67-6 = 17-8 



Hg] 22-4 - 11-9 = 10-5 



Ag] 19-7 - 3-6 = 16-1 



Also between cyanide and iodides (I gas) 



Difference. 



K] 100-4 + 67-5 = 32-8 



Hg] 30-4 + 11-9 = 18-1 



27-7 + 3-6 = 24-1 



[K] 

 [Hg] 



[Ag] 



These inequalities result from- the great quantity of heat 

 disengaged by the union of the hydrocyanic acid with metallic 

 oxides as compared with the small quantity disengaged in the 

 union of the same acid with alkalis. 



9. DOUBLE CYANIDES. 



It is now necessary to find the heat of formation of double 

 cyanides, such as the cyanides of mercury and potassium, silver 

 and potassium, and also that of ferrocyanides, which are worthy 

 of particular attention. 



1. Cyanide of mercury and potassium : Hg(CN 2 ), 2KCN. This 

 compound offers a remarkable example of a double salt which 

 exists and is even undoubtedly generated in solutions. In fact, 

 it was found that its two components, when in solution, give off 

 a great quantity of heat if merely mixed together 



-J[Hg(CN) a (1 equiv. = 16 litres) + 2KCN (1 equiv. = 4 litres)] 



+ 5-8. 



This quantity represents nearly two-thirds of the heat dis- 

 engaged by the union of the two salts when in the solid state. 

 The latter value is calculated by means of the following data : 



KCN, on being dissolved in 40 times its weight of water 2-96 



i[Hg(CN) 2 ], on being dissolved in 40 times its weight of water ... 1-60 

 i[2KCN, Hg (ON) J, on being dissolved in 40 times its weight of water 6'96 



These data go to show that the combination 



J[Hg(CN) 2 (dry) + 2KCN (dry) = Hy(CN) 2 , 2KCN (dry)], 

 disengages -f 8 -3, 



which is a considerable quantity of heat. It approaches, and 

 even exceeds, the heat disengaged in the formation of many 

 metallic salts, from the anhydrous acid and base. However, 

 the double cyanide, in solution, is immediately decomposed by 

 diluted hydrochloric acid, with separation of its components ; 

 the mercuric cyanide being regenerated unaltered in the liquor, 

 and the potassium cyanide being converted into potassium 

 chloride. 



This was discovered by measuring the heat disengaged in the 



