olour, causes the more rapid absorption of heat from the sun's rays. It
will be thus understood, that though it does not directly supply food to
the plant, it ministers indirectly in a most important manner to its
well-being, and that to so great an extent that it must be considered an
indispensable constituent of a fertile soil. But it is important to
observe that it must not be present in too large a quantity, for an
excess does away with all the good effects of a smaller supply, and
produces soils notorious for their infertility.

Such are the important physical properties of the soil, and it is
greatly to be desired that they should be more extensively examined. The
great labour which this involves has, however, hitherto prevented its
being done, and will, in all probability, render it impossible except in
a limited number of cases. Some of these characters are, however, of
minor importance, and for ordinary purposes it might be sufficient to
determine the specific gravity of the soil in the dry and moist state,
the power of imbibing and retaining water, its hygrometric power, its
tenacity, and its colour. With these data we should be in a condition to
draw probable conclusions regarding the others; for the higher the
specific gravity in the dry state, the greater is the power of the soil
to retain heat, and the darker its colour the more readily does it
absorb it. The greater its tenacity the more difficult is it to work,
and the greater difficulty will the roots of the young plant find in
pushing their way through it. The greater the power of imbibing water,
the more it shrinks in drying; and the more slowly the water evaporates,
the colder is the soil produced. The hygrometric power is so important a
character that Davy and other chemists have even believed it possible to
make it the measure of the fertility of a soil; but though this may be
true within certain limits, it must not be too broadly assumed, the
results of recent experiments by no means confirming the opinion in its
integrity, but indicating only some relation between the two.

_The Subsoil._--The term soil is strictly confined to that portion of
the surface turned over by the plough working at ordinary depth; which,
as a general rule, may be taken at 10 inches. The portion immediately
subjacent is called the subsoil, and it has considerable agricultural
importance, and requires a short notice. In many instances, soil and
subsoil are separated by a p