WATER. 
ted hydrogen gas have demonstrated, that air, even af 
ter it has been exposed to the action of the driest salts 
and those which have the strongest affinity for moisture, 
still retains a portion of water ; but this portion is ex 
ceedingly small, and may, when the precautions of Saus 
sure are taken, be neglected altogether without any sen 
sible error. Mr Henry has pointed out a way of free 
ing some of the gases altogether from water by the ac 
tion of electricity. When they are thus completely pu 
rified from water, they continue to retain their elastic 
form as much as ever. We may conclude, then, that 
air always contains water, but that water is not an in 
gredient so essential as to be necessary to the very exist 
ence of air in an elastic form; though a certain portion 
of it seems essential to render air fit to answer the diffe 
rent purposes for which it is intended ; very dry air 
having exceedingly unpleasant effects upon the animal 
economy, as has been remarked by Saussure and other 
philosophers. We may conclude also, that the abso 
lute quantity of water assigned by Saussure as contain 
ed in air, corresponding to the different degrees of his 
hygrometer, does not deviate very far from the truth. 
With respect to the state in which water exists in 
air, there are only three different suppositions possible, 
each of which has been supported by very able philo. 
sophers. 1. Water may be dissolved in air in the same 
manner as'a salt is held in solution by water 2. It 
may be mechanically mixed with air in the state of 
steam or vapour ; or, 3. It may be chemically combi 
ned with air, after having been converted into vapour. 
1. The first of these opinions was hinted at by Dr 
Hooke in his Micrographia, and afterwards proposed 
by Dr Halley; but it was much more fully developed 
S 3 
277 
Chap. I. 
State in 
which wa 
ter exists ir 
air.