458 The Atmosphere Considered ’ October, - 
3. Water through which air is passed dissolves the two 
gases in very different proportions to those in which 
they are associated, the oxygen being very soluble, 
while the nitrogen is not taken up to any notable 
extent. 
Carbonic Acid . 
Although the bulk of the atmosphere is made up of the 
two gases just referred to, these do not take so active shares 
in geological matters as the almost infinitesimal trace of 
carbonic acid present. This, then, deserves the place of 
honour in the following pages, and it will be seen that there 
is a great deal to be said about it. We shall therefore defer 
the consideration of the behaviour of the other constituents 
for a little while. 
The amount of carbonic acid ranges from about 3 to 
10 volumes in 10,000 volumes of air, and the proportion 
varies between these limits in different localities, owing to 
many modifying causes. In the neighbourhood of towns or 
cities it will be much increased by the combustion of fuel, 
the exhalations of animal life, and the decay of organic 
matters. In the vicinity of large forests, swamps, and fens, 
vegetable decay will also augment it, though at the same 
time the living vegetation there will help to re-absorb it, or, 
to speak exactly, to decompose it. Near volcanoes the air 
will be more or less impregnated with it ; and from many 
mineral springs, and subterranean caves and fissures, a very 
considerable quantity of this gas is discharged into the at- 
mosphere. The percentage of carbonic acid also varies 
slightly between day and night. 
Geological Effects . 
So small a trace as even 10 in 10,000— taking the maxi- 
mum, only o*i per cent — certainly does not at first sight 
seem capable of performing any very great geological work; 
but we must recoiled that the vast quantities of existing 
vegetation are entirely dependent on the carbon they obtain 
from the atmosphere, and the decay of vegetation, and con- 
sequent liberation of carbonic acid, has a very powerful 
effeCt in the alteration or solution of rocks. However, the 
direCt aCtion of atmospheric carbonic acid on rocks — both 
as a destructive and as a recuperative agent — must be any- 
thing but small, even at the present day. As to the latter, 
it is only necessary to refer to the immense coral reefs now 
being formed, while the widespread deposits of ooze and 
mud over the floors of the Atlantic and Pacific are largely 
