attraction is evaporated, and
hence the soil is more frequently and for a longer period in a condition
to take advantage of the heating effect of the sun's rays, and in this
way the period of germination, and, by consequence also, that of
ripening is advanced. The extent of this influence is necessarily
variable, but it is generally considerable, and in some districts of
Scotland the extensive introduction of draining has made the harvest, on
the average of years, from ten to fourteen days earlier than it was
before. It is unnecessary to insist on the importance of such a change,
which in upland districts may make cultivation successful when it was
previously almost impossible. The removal of moisture by drainage
affects the physical characters of the soil in another manner; it makes
it lighter, more friable, and more easily worked; and this change is
occasioned by the downward flow of the water carrying with it to the
lower part of the soil the finer argillaceous particles, leaving the
coarser and sandy matters above, and in this way a marked improvement is
produced on heavy and retentive clays. The access of air to the soil is
also greatly promoted by draining. In wet soils the pores are filled
with water, and hence the air, which is so important an agent in their
amelioration, is excluded; but so soon as this is removed, the air is
enabled to reach and act upon the organic matters and other decomposable
constituents present. In this way also provision is made for the
frequent change of the air which permeates the soil; for every shower
that falls expels from it a quantity of that which it contains, and as
the moisture flows off by the drains, a new supply enters to take its
place, and thus the important changes which the atmospheric oxygen
produces on the soil are promoted in a high degree. The air which thus
enters acts on the organic matters of the soil, producing carbonic acid,
which we have already seen is so intimately connected with many of its
chemical changes. In its absence the organic matters undergo different
decompositions, and pass into states in which they are slowly acted on,
and are incapable of supplying a sufficient quantity of carbonic acid to
the soil; and they thus exercise an action on the peroxide of iron,
contained in all soils, reduce it to the state of protoxide, or, with
the simultaneous reduction of the sulphuric acid, they produce sulphuret
of iron, forms of combination which are well know