OF SOME SALTS OF SODA. 93 
hence then I consider that when the vapour 
point is only at 49°, the disposition of water to 
evaporate, or in other words, the affinity of 
space for vapour, is just equal to the affinity of 
the salt for its water of crystallization—that 
then the crystallized salt would not lose water, 
neither would the proto-hydrate gain water. 
I have also stated that crystals of sulphate of 
soda only begin to effloresce at the temperature 
of 58° when the vapour point is at 49°: hence I 
consider that when the vapour point is only at 
50° the affinity of space for vapour is just equal 
to the affinity of this salt for its water of crys- 
tallization. 
Now, by Dr. Dalton’s table* we find that the 
force of vapour at 58° is equal to 0.62 of an 
inch, and at 50° to 0.49 of aninch. Then, by 
deducting 0.47 from 0.62 we have 0.15 of an 
inch of mercury—the force of affinity with which 
carbonate of soda holds its water of crystalli- 
zation. 
By adding 0.15 to 0.62 we get 0.77, which 
we find by the table is the force of vapour at 
* T refer to that table in the ii. vol. of his System of Che- 
mistry. 
