326 HEATS OF FORMATION OF THE CYANOGEN SERIES. 



reaction. This measurement proves, in fact, that dilute hydro- 

 chloric acid, acting on the solution of the double cyanide, 

 separates the components, with reproduction of potassium 

 chloride and hydrocyanic acid : 



[iHg(CN)2, 2KCN (20 litres) + 2HC1 (1 equiv. = 2 litres], + 5*2 



-4HC1 +0 



The calculation, based upon these last data, indicates the 

 following value: -f 3'0 -f 5'8 + 5'2 = + 14;0 for the heat dis- 

 engaged in the union of hydrochloric acid with potash ; a value 

 which agrees to all intents with the actual value + 13 '6, that 

 is, if the liquors are as much diluted as in the above. 



2. Cyanide of silver and potassium : AgCN, KCN. This salt, 

 so much used in electro-plating, acts in a manner similar to the 

 above. It is formed by the direct action of potassium cyanide 

 in solution on precipitated silver cyanide, the latter becoming 

 dissolved with disengagement of heat 

 KCN (1 eq. = 4 litres) + AgCN" (precipitated) -f water (20 litres) 



gives off -f 5*6. 



The reaction disengages almost the same quantity of heat as 

 that in the case of mercuric cyanide, notwithstanding the solid 

 state of the silver cyanide. 



This is a fresh instance of the solution of a precipitate being 

 effected with disengagement of heat, in consequence of the for- 

 mation of a double salt. The phenomena are dependent on this 

 formation, independently of the solubility or insolubility of the 

 original metallic cyanide (mercuric or silver), as the double salt 

 is formed with disengagement of heat, and is stable in the 

 presence of excess of the dissolving agent. 



It was also found that the solution of the soluble salt 



AgCN, KCN (solid) (1 part = 40 parts of water), absorbs - 8'55. 



"We conclude from these data, and also the heat of solution of 

 potassium cyanide, that the combination 



AgCN (precipitated) + KCN (dry) = AgCN, KCN" (dry), 

 disengages + 11 '2. 



The double salt in solution is immediately decomposed by 

 dilute hydrochloric acid, with reproduction of potassium 

 chloride and hydrocyanic acid, as is proved by thermal 

 measurements. At the same time a precipitate of silver 

 chloride is produced, mixed with a considerable proportion of 

 cyanide, as might be expected ; for the formation of both salts 

 from the diluted hydracids and precipitated silver oxide causes 

 the liberation of about the same quantity of heat (-f 20'9). 



The double cyanide of silver and potassium, however, con- 

 stitutes a firmer combination than is usually met with in 

 ordinary double salts. In fact, dilute acetic acid separates 

 silver cyanide from it only in a very incomplete manner, giving 



