The crevice water of the earth, although forming at no time more than
a very small fraction of the hidden fluid, is an exceedingly potent
geological agent, doing work which, though unseen, yet affords the
very foundations on which rest the life alike of land and sea. When
this water enters the earth, though it is purified of all mineral
materials, it has already begun to acquire a share of a gaseous
substance, carbonic acid, or, as chemists now term it, carbon dioxide,
which enables the fluid to begin its rôle of marvellous activities. In
its descent as rain, probably even before it was gathered in drops in
the cloud realm, the water absorbs a certain portion of this gas from
the atmosphere. Entering the realm of the soil, where the decaying
organic matter plentifully gives forth carbon dioxide, a further store
of the gas is acquired. At the ordinary pressure of the air, water may
take in many times its bulk of the gas.

The immediate effect of carbonic acid when it is absorbed by water is
greatly to increase the capacity which that fluid has for taking
mineral matters into solution. When charged with this gas, in the
measure in which it may be in the soil, water is able to dissolve
about fifty times as much limestone as it can in its perfectly pure
form take up. A familiar instance of this peculiar capacity which the
gas gives may often be seen where the water from a soda-water fountain
drips upon the marble slab beneath. In a few years this slab will be
considerably corroded, though pure water would in the same time have
had no effect upon it.

The first and by far the most important effect of crevice water is
exercised upon the soil, which is at once the product of this action,
and the laboratory where the larger part of the work is done.
Penetrating between the grains of the detrital covering, held in large