The aqueous vapor condensed from the air dissolves
part of the carbonic acid contained therein, and carries it along, when
it falls as rain upon the earth, and takes up there enough lime to form
the bicarbonate, which is thus carried back to the sea.
The physiological role of carbonic acid, its geognostic influence, and
its relations to most ordinary meteorological phenomena on the earth's
surface--all these contribute to give special weight to studies
concerned in the estimation of the normal quantity of carbonic acid in
the air.
Nevertheless, this estimation is attended with great difficulty. Not
everyone is able to take up such questions, and not all processes are
adapted to it. The first thought which would naturally arise would be to
inclose a known volume of air in a given vessel, and then determine its
carbonic acid by measuring or weighing it. In this way we should obtain
the exact relation between a volume of air and the volume of carbonic
acid in it, for any given moment, and in any given place. If, however,
this be done with a ten-liter flask, for example, it would only hold
3 c.c. of carbonic acid, weighing 6 milligrammes; and, whether it is
weighed or measured, the error may easily equal 10 per cent.
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