h it exists, so as to have some idea
of the ease or difficulty with which they may be absorbed. For this
purpose it is necessary to determine, _1st_, The substances soluble in
water; _2d_, Those insoluble in water, but soluble in acids; _3d_, Those
insoluble both in water and acids; and if to these the organic
constituents be added, there are four separate heads under which the
components of a soil ought to be classified. This classification is
accordingly adopted in the most careful and minute analyses; but the
difficulty and labour attending them has hitherto precluded the
possibility of making them except in a few instances; and, generally
speaking, chemists have been contented with treating the soil with an
acid, and determining in the solution all that is dissolved. Such
analyses are often useful for practical purposes, as for example, when
they show the absence of lime, or any other individual substance, by the
addition of which we may rectify the deficiency of the soil; but they
are of comparatively little scientific value, and throw but little light
on the true constitution of the soil, and the sources of its fertility.
Nor is it likely that much satisfactory information will be obtained
until the number of minute analyses is so far extended as to establish
the fundamental principles on which the various properties of the soil
depends.

The separation of the constituents of a soil into the four great groups
already mentioned, is effected in the following manner:--A given
quantity of the soil is boiled with three or four successive quantities
of water, which dissolves out all the soluble matters. These generally
amount to about one-half per cent of the whole soil, and consist of
nearly equal proportions of organic and inorganic substances. In very
light and sandy soils, it occasionally happens that not more than one or
two-tenths per cent dissolve in water, and in peaty soils, on the other
hand, the proportion is sometimes considerably increased, principally
owing to the abundance of soluble organic matters.

When the residue of this operation is treated with dilute hydrochloric
acid, the matters soluble in acids are obtained in the fluid. The
proportion of these substances is liable to very great variations, and
in some soils of excellent quality, and well adapted to the growth of
wheat, it does not exceed 3 per cent; while in calcareous soils, such as
those of the chalk formation, it may reach as much as 50 or