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467 
in its Geological Relations . 
of existence; but it is undeniable that such would be the case. 
The blood requires to be oxygenated, but in the absence of 
carbon there would be no blood at all. And this leads us to 
another point. The disappearance of carbonic acid must be 
followed after a period by the withdrawal of oxygen itself. 
It would gradually be carried by water into the interior of 
the earth, from which it could make no return, for it would 
be seized upon by compounds capable of oxidation, and 
its retreat in the form of carbonic acid would have been 
cut off. 
As to the first part of the question, however, we have as 
yet no data for its solution. There are several means by 
which carbonic acid is supplied to the air, and many by 
which it is removed ; but we are not in a position to deter- 
mine on which side is the predominance, or whether there 
is at present a balance of power. The principal sources of 
increase are — 
1. Volcanic and other subterranean exhalations. 
2. Respiration of animals. 
3. Combustion of fuel, & c. 
Respecting this last it should be pointed out that we are 
now restoring to the atmosphere some of the vast quantities 
of carbonic acid abstracted from it during the Carboniferous 
period, and imprisoned for ages in the interior of the earth 
in the forms of coal and clay-ironstone. Perchance by the 
time we have made an end of our supplies of coal a very 
sensible difference will have been effected in our atmo- 
sphere. 
The absorption of the carbonic acid is brought about 
thus : — 
1. By vegetation, as already explained. 
2. By the agency of marine organisms which secrete car- 
bonate of lime. 
3. By the direCt aCtion of atmospheric carbonic acid upon 
rocks, resulting in the formation of carbonates. 
How far these antagonistic processes check each other can- 
not be conjectured. In order to arrive at any conclusion on 
the matter we should require to compare trustworthy 
analyses of air taken at frequent intervals during some 
thousands of years at least. We have yet no recorded 
analyses of it older than forty or fifty years. Probably in 
the remote future information will have been accumulated 
sufficiently to allow of the solution of the problem ; and 
perhaps in those far distant times a Royal Commission, or 
some such form of Public Inquiry, will be solemnly convened 
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