780 Proceedings of the Royal Society 
heat of solution, and the greater the affinity between the positive 
and negative elements the less is the heat of solution. Whether 
the quantity of salt dissolved varies exactly at same rate, or only 
proportionately, I am not yet in a position to state, hut it is 
generally true that the greater the heat of solution the greater is 
the amount of salt dissolved in similar compounds. The following 
examples illustrate the laws just stated — 
Compound. 
Heats of 
Formation. 
Difference. 
Heat of Solu- 
tion of 
Chlorides. \ 
Difference. 
[Mg,Cl 2 ] 
151010 
35920 
[Mg,0,Aq] 
148960 
- 
2050 
[Ca,cy 
169820 
17410 
[Ca,0,Aq] 
149260 
20560 
-18510 
18510 
[Ca,Cl 2 ] 
169820 
17410 
[Ca,0,Aq] 
149260 
20560 
[Sr, Cl 2 ] 
184550 
11140 
[Sr, 0, Aq] 
157780 
26770 
-6210 
6270 
[Sr,ClJ 
184560 
11140 
[Sr,0,Aq] 
157780 
26770 
[Ba,Cl 2 ] 
194740 
2070 
[Ba,0,Aq] 
158760 
85980 
-9210 
9070 
Exactly similar results are obtained with the bromides and iodides, 
and also with the chlorides, bromides, and iodides of the alkali 
metals. There is, however, a variation in the case of metals which 
form insoluble oxides or hydrates. In the latter case the heats of 
solution of chlorides, &c., are not so great as they should he if com- 
pared with above compounds. Among themselves, however, they 
follow the above law pretty closely, and seem arranged in groups. 
Thus, ZnCl 2 and CdCl 2 , and EeCl 2 CoCl 2 and MC1 2 form two such 
groups. 
In the foregoing illustrations I have shown the effects of the 
