MERCURIC CYANIDE. 319 



less heat (3*0), is displaced by mercuric oxide. On the other 

 hand, the solution of this salt 



[Hg(CN) 2 (solid) + water (1 to 40)] -1-5 ; 

 consequently, 



[2HCN (in solution) + HgO = Hg(CN) 2 (solid)] ... + 17'0 

 M2HCN (liquid and pure) + HgO = Hg(CN) 2 (solid)] + 17-4 

 [2HCN (gas) + HgO = Hg(CN) 2 (solid) + H 2 (gas)] + 18'3 



We will compare this last result with the heat of formation of 

 mercuric chloride. 



j[2HCl (gas) + HgO = HgCl 2 (solid) + H 2 (gas)] gives off 



+ 23-5, 



which value exceeds that of formation of mercuric cyanide by 

 4-8 only. 



These figures, and the conclusions resulting from them, will 

 be referred to again. 



2. Formation of mercuric cyanide from the elements. 



l[Hg (liq.) + C 2 (diamond) + N 2 (gas) = Hg(CN) 2 (solid)] 

 absorbs - 254 Gal. 



Let the initial system be 



and the final system 



i [Hg(CTST) 2 (solid) + H 2 (liquid and separate)] ; 

 we pass from one to the other, by the two following steps : 



FIRST STEP. 



*(Hg + = HgO) gives off + 15-5 



fl + c+ N= HCN (in solution) absorbs - 23-4 



Union of these two bodies gives off ... ... ... + 15'5 



Separation of Hg(CN) 2 (solid) gives off + 1-5 



Sum ... +9-1 

 SECOND STEP. 



tH 2 + = H 2 (liquid)] ... +34*5 

 Hg + C 2 + N 2 = Hg(CN) 2 (solid)] x 



Sum ... + 34-5 -f x 

 x = - 25-4. 

 The salt in solution, - 26-9. 



There is, therefore, absorption of heat in the formation of 

 mercuric cyanide from the elements ; exactly as in the case of 

 hydrocyanic acid. The figures even are very much the 

 same (p. 312). 



3. But, on the contrary, the union of gaseous cyanogen with 

 liquid mercury at the ordinary temperature 



i[Hg (liquid) + (CN a ) (gas) = Hg(CN) 2 (solid)], gives off 

 + 37-3 - 25-4 = + H-9. 



