f 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. Let us now, therefore, trace the effect of this
arrangement. In Fig. 1 we perceive that these canals and pores are
all empty, the soil being _perfectly dry_; and the canals
communicating freely at the surface with the surrounding
atmosphere, the whole will of course be filled with air. If in
this condition a seed be placed in the soil, at _a_, you at once
perceive that it is freely supplied with air, _but there is no
moisture_; therefore, when soil is _perfectly dry_, a seed cannot
grow.

[Illustration: Fig. 2 - A WET SOIL.]

Fig. 2 - A WET SOIL.

"Let us turn our attention now to Fig. 2. Here we perceive that
both the pores and canals are no longer represented white, but
black, this color being used to indicate water; in this instance,
therefore, water has taken the place of air, or, in other words,
the soil is _very wet_. If we observe our seed _a_ now