48 
Proceedings of Royal Society of Edinburgh. [jan. 17, 
When the masses are unequal it is shown that the temperature 
must he kept constant to insure a steady state of diffusion. 
3. Intimation of an Improvement in Rankine’s Formula for 
Retaining Walls. Given by Professor Armstrong on 
behalf of Mr Elliott. 
Monday, Vlth January 1887. 
SHERIFF IRVINE, Vice-President, in the Chair. 
The following Communications were read : — 
1. The Total Rainfall on the Land of the Globe, and its 
Relation to the Discharge of Rivers. By J. Murray, 
Esq., Ph.D., V.P. 
2. Chemical Affinity and Solution. By W. Durham, Esq. 
In continuation of my inquiry into the evidence which thermo- 
chemistry gives of the truth of my theory of chemical affinity and 
solution, I would direct attention to the sulphates. 
In the first instance, consider the well-known definite compound 
sulphuric acid H 2 S0 4 . The heat which is evolved on building up 
this acid from its elements is 192920 units. When it is dissolved 
in a large quantity of water the mixture evolves 17850 units, 
making in all 210770 units. Now, consider how this is made up. 
First, we have H 2 0 with a combination heat of 68360 units, then 
S0 3 with 103240 units. Further, we know that S combines with 
H 2 , evolving 4740 units of heat, and let us assume, according to 
my theory, that all the affinity of the S for O is not exhausted on 
the combination S0 3 , but that part remains in a less intense form 
which can act on the O of the water, and in conjunction with H 2 can 
evolve 34413 units more, this being its average action on the 
three atoms of O already combined. Add these numbers together, 
