220 Prof. E. F. Herroun on the Divergence of 



reaction which occurs between mercurous nitrate, free nitric 

 acid, and SO2 one may reasonably expect to be less simple 

 than that required for exact thermal measurement. 



I have accordingly determined the heat of formation of 

 mercurous nitrate in acid solution, by decomposing it with 

 zinc amalgam, yielding zinc nitrate and metallic mercury ; 

 the results obtained are subjoined. 



Mercury used. Heat-units cooled. Heat of Reaction. 



1-362 grms. 265-5 grms.°C. 77,975 



1-362 „ 206-2 „ 78,180 



2-58 „ 492-1 „ 76,300 



Mean = 77,485 



Since Thomsen finds 102^510 for [Zn, 0, N2O5 Aq], the 

 heat of formation of 



[Hg2,0, N205Aq] = 102,510- 77,485 = 25,025 calories. 



If this be approximately true, it follows that the values 

 given by Thomsen for the heats of formation of the chloride, 

 bromide, and iodide of mercury must be greatly in excess of 

 the true value. The heat of formation of [Hgo, 012] is given 

 by him as 82,550, taking 47,990 as the value for the nitrate. 

 But it is well known that calomel is reduced to metallic 

 mercury by an acid solution of stannous chloride, although 

 the .heat evolved in the conversion of 8nCl2Aq into SnCl4Aq 

 is only 76,030. Hence splitting up Hg2Cl2 with the forma- 

 tion of stannic from stannous chloride would give, according 

 to Thomsen, the following thermal equation : — 



[SnCl2,Cl2,Aq1-[Hg2,Cl2] 



76,030 '- 82,550 =-6520, 



so that this reaction would be markedly endothermic and 

 could not well occur at ordinary temperatures. 



The matter, however, was put to experimental test, and it 

 was found that although the reaction was slow, requiring 

 about three hours for its completion at a temperature of 

 about 1 5° C, still heat was continuously evolved, and, after 

 calculating for loss of heat, gave the following values : — 



Weiglit of Hg,Cl2. Heat evolved. Heat of Reaction. 



5-067 grnis. 120-76 grms.° C. 11,225 



4-783 „ 104-8 „ 10,325 



Mean = 10,775 

 Griving 76,030 — 10,775 = 65,255 calories as the heat of forma- 

 tion of HggCls. 



The electromotive forces calculated from these thermal 



