1887 .] 
Mr W. Durham on Chemical Affinity. 
51 
Now this exceeds 84200 by 4423, which is the heat of solution, 
viz., 4440, and accounts for so much of the difference ; hut the 
remarkable thing now comes in, CaS0 4 combines with 2H 2 0, and 
in doing so evolves 300 units of heat more than the heat of solu- 
tion, viz., 4740, which is exactly the heat of combination of SH 2 . 
We see, therefore, that owing to the whole affinity of the S for Ca 
not being exercised in the compound CaS0 4 , the S can exercise its 
normal affinity towards H 2 of the water, and as a consequence we 
have CaS0 4 slightly soluble, while the other two analogous salts are 
insoluble. Further, when this compound CaS0 4 2H 2 0 is dissolved 
in water, the excess of its heat above that of solution appears as 
a negative quantity, therefore its heat of solution is - 300. Now 
this admirably illustrates the meaning of these negative heats of 
solution, and also two points to which I drew attention : — First, 
the lowering of intensity of affinity. We have in this case affinity 
represented by 300 units of so low a tension that its presence can 
only he detected when acting on two molecules of water. On a 
larger quantity it has no effect so far as temperature is concerned. 
The second point is that every molecule of water exercises affinity 
on every other molecule, but as the work done and undone must be 
equal everywhere, there is no change of temperature ; but it is 
entirely different if one or two molecules be bound to another 
foreign body; the balance is then upset, and the result will be a 
change of temperature in one direction or the other. There are 
still about 3000 units of heat to account for. Now, I am not pre- 
pared to say exactly where these will be found. According to my 
theory the affinity of the S is now so reduced in intensity that it 
cannot make its presence known by evolution of heat in the ordi- 
nary way. It may be, however, that here we have the explanation 
of the facts pointed out in my former papers regarding the precipi- 
tation of clay suspended in water, by the addition of a very small 
quantity of a soluble salt. 
Take one more example as extremely interesting for the fresh 
light it throws on the subject — consider Na 2 S0 4 . In building this 
compound up from its elements 328590 units of heat are evolved, 
and in addition the compound combines with ten molecules of 
water with an additional evolution of 19220 units. Proceeding as 
in the other cases we have — 
