s conditions of soil, for
the purpose of determining how far these will influence the future
prospects of the crop, and we shall accordingly at once proceed to
examine carefully into the _mechanical relations of the soil_. This
we propose doing by the aid of figures. Soil examined mechanically,
is found to consist entirely of particles of all shapes and sizes,
from stones and pebbles, down to the finest powder; and, on account
of their extreme irregularity of shape, they cannot lie so close to
one another as to prevent there being passages between them, owing
to which circumstance soil in the mass is always more or less
_porous_. If, however, we proceed to examine one of the smallest
particles of which soil is made up, we shall find that even this is
not always solid, but is much more frequently porous, like soil in
the mass. A considerable proportion of this finely-divided part of
soil, _the impalpable matter_ as it is generally called, is found,
by the aid of the microscope, to consist of _broken-down vegetable
tissue_, so that when a small portion of the finest dust from a
garden or field is placed under the microscope, we have exhibited
to us particles of every variety of shape and structure, of which a
certain part is evidently of vegetable origin. In these figures I
have given a very rude representation of these particles; and I
must beg you particularly to remember that they are not meant to
represent by any means accurately what the microscope exhibits, but
are only designed to serve as a plan by which to illustrate the
mechanical properties of the soil. On referring to Fig. 91, we
perceive that there are two distinct classes of pores; first, the
large ones, which exist _between_ the particles of soil, and
second, the very minute ones, which occur in the particles
themselves; and you will at the same time notice, that whereas all
the larger pores--those between the particles of soil--communicate
most freely with each other, so that they form canals, the small
pores, however freely they may communicate with one another in the
interior of the particle in which they occur, have no direct
connection with the pores of the surrounding particles. Let us now,
therefore, trace the effect of this arrangement. In Fig. 91, we
perceive that these canals a