exerts a strong solvent action, but when it has been rendered impure
by a variety of substances, naturally occurring, its solvent action
is greatly increased.

The most effective water impurity, considering soil formation, is
the gas, _carbon dioxid. _This gas is formed whenever plant or
animal substances decay, and is therefore found, normally, in the
atmosphere and in soils. Rains or flowing water gather the carbon
dioxid from the atmosphere and the soil; few natural waters are free
from it. The hardest rock particles are disintegrated by carbonated
water, while limestones, or rocks containing lime, are readily
dissolved.

The result of the action of carbonated water upon soil particles is
to render soluble, and therefore more available to plants, many of
the important plant-foods. In this way the action of water, holding
in solution carbon dioxid and other substances, tends to make the
soil more fertile.

The second great chemical agency of soil formation is the oxygen of
the air. Oxidation is a process of more or less rapid burning, which
tends to accelerate the disintegration of rocks.

Finally, the _plants _growing in soils are powerful agents of soil
formation. First, the roots forcing their way into the soil exert a
strong pressure which helps to pulverize the soil grains; secondly,
the acids of the plant roots actually dissolve the soil, and third,
in the mass of decaying plants, substances are formed, among them
carbon dioxid, that have the power of making soils more soluble.