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.
[Illustration: Fig. 1 - A DRY SOIL.]
Fig. 1 - A DRY SOIL.
"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. 1, 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.
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