les.
These show the presence of bacteria, which convert the atmospheric
nitrogen into a form in which the plants can use it. Scientific farmers
have learned the value of inoculating their soil with these germs. A
crop of peas or clover may produce the same result.

4. Observe Nature's method of supplying soil with humus.

SOIL-FORMING AGENTS

There was once a time when the surface of the earth was bare rock. Much
of this rock still exists and in many places lies on the surface, but it
is usually hidden by a layer of soil. Soil is said to be "rock ground to
meal by Nature's millstones". The process is very slow, but it is
constantly going on. The pupils should be directed to find evidences of
this "grinding".

1. RUNNING WATER.--Brooks, creeks, rain, and the tiny streamlets on the
hills all tell us how soil is carried from place to place. Get some
muddy water from the river after a heavy rain. Let it settle in a tall
jar and observe the fine layer formed.

Wash some pebbles clean, place them in a glass jar with some clear
water, and roll or shake the jar about for a few minutes. Note that the
water becomes turbid with fine material worn from the stones. A process
similar to this is constantly going on in rivers, lakes, and seas.
Account for the presence of gravel beds now situated far away from any
water.

2. ICE GLACIERS.--How do these act on rocks? Show evidences in Ontario
as far as these can be illustrated from the surroundings, such as
polished rocks, boulders, beds of clay, sand, or gravel, small lakes,
grooved stones, etc.

3. FROST AND HEAT.--See "Expansion of Solids", pages 189, 190. Look for
splintered or cracked stones. Why do farmers plough in the fall?

4. WIND.--In sections near the lakes the action of the wind in moving
the sand may be seen and appreciated. There are other places where this
work is going on on a smaller scale.

5. PLANTS.--Our study of humus shows the value of vegetable matter in
soil. Besides contributing to the soil, plants break up rocks with their
roots and dissolve them with acid excretions. It is interesting to study
how a bare rock becomes covered with soil. First come the lichens which
need no soil; on the remains of these the mosses grow. The roots of
mosses and lichens help to disintegrate the rock with their excretions,
so that, with frost, heat, air, and rain to assist, there is a layer of
soil gradually formed on which larger plants can live. A forest
develops. The