574 On the Force of Steam or Vapour 
librium continues.—In short, in all cases the 
vapour arises to a certain force, according to 
temperature, and the air adjusts the equilibrium, 
by expanding or contracting as may be required. 
The notion of a chemical affinity subsisting be- 
tween the gases and vapours of different kinds, 
cannot at all be reconciled to these phenomena. 
To suppose that all the different gases have the 
same affinity for water might indeed be admitted 
if we could not explain the phenomena without 
it; but to go further, and suppose that water 
combines with every gas’ to the same amount 
as its vapour in vacuo; or in other words, that 
the elasticity of the compound should be exactly 
the same as if the two were separate, is certainly 
going far to serve an hypothesis. 
Besides, we must on this ground suppose that 
all the gases have the same force of affinity 
for any given vapour; a supposition that 
cannot be admitted as having any analogy to 
other established Jaws of chemical affinity. 
ESSA¥ , ILI. 
On Evaporation. 
When a liquid is exposed to the air, it be- 
comes gradually dissipated in it: the process 
by which this effect is produced, we call evapo-< 
ration. 
