1887.] Mr W. Durham on Laws of Solution. 385 
Perfectly analogous results are obtained on changing the positive 
element of the salt instead of the negative. In every case we find 
the heat of solution regulated by the chemical affinities (as measured 
by heat) of the elements. Another instructive instance of the rela- 
tions of chemical affinity and solution is found in the double salts 
of the form MS0 4 , R"S0 4 , 6H 2 0, where M forms crystals of the 
composition MS0 4 7H 2 0. In the double salts E/'S0 4 takes the 
place of one molecule of H 2 0, and develops more or less heat in so 
doing. How the thermal results of solution of the double salt seems 
to indicate that decomposition is brought about. 
Consider the following : — 
Heats of Combination. 
[ZnS0 4 ,K 2 S0,6H 2 0] = 23950 
[ZnS0 4 ,7H 2 0] = 22690 
Difference = + 1260 
Heats of Solution. 
[ZnS0 4 K 2 S0 4 6H 2 0 Aq] =-11900 
[ZnS0 4 7H 2 0,Aq] = - 4260 
[K 2 S0 4 ,Aq] - 6380 
- 10640 
Difference - 1260 
In fact, putting the double crystalline salt into solution brings the 
mixture to exactly the same thermal state as if the constituent 
sulphates were separately dissolved in water. 
{Added July 16, 1887.) 
It has been said that no argument as to residual affinity can be 
based on thermal results because we do not know the fundamental 
units, but it appears to me there is no force whatever in this objec- 
tion, as we are not dealing with absolute affinity but only with 
differences, and thermal chemistry is particularly fitted to show these 
differences. Thus, for instance, Cl in combining with Sr develops 
10190 units less heat than it does when combining with Ba. How 
the question is, What becomes of these 10190 units'? Are they lost 
entirely, or is there residual affinity left in SrCl 2 to that amount % 
