ish tinge. These holes are useful because they let air and water
down into the soil.

[Illustration: Fig. 24. Soil in which earthworms have been living and
making burrows]

The following experiment shows what earthworms can do. Fill a pot with
soil from which all the worms have been carefully picked out and
another {55} with soil to which earthworms have been added, one worm to
every pound of soil. Leave them out of doors where the rain can fall
on to them. You can soon see the burrows and the heaps of soil or
"casts" thrown up by the worms: these casts wash or blow over the
surface of the soil, continually covering it with a thin layer of
material brought up from below. Consequently the soil containing
earthworms always has {56} a fresh clean look. After some time the
other soil becomes very compact and is covered with a greenish slimy
growth. When this happens carefully turn the pots upside down, knock
them so as to detach the soil and lift them off. The soil where the
earthworms had lived is full of burrows and looks almost like a sponge.
Fig. 24 shows what happened in an experiment lasting from June to
October. The other soil where there were no earthworms shows no such
burrows and is rather more compact than when it was put in.

Earthworms therefore do three things:--

(1) They make burrows in the ground and so let in air and water.

(2) They drag leaves into the soil and thus help to make the mixture of
soil and leaf mould.

(3) They keep on bringing fresh soil up to the surface, and they
disturb the surface so much that it is always clean and free from the
slimy growth.

All these things are very useful and so a gardener should never want to
kill worms. The great naturalist, Darwin, spent a long time in
studying earthworms at his home in Kent and wrote a very interesting
book about them, called _Earthworms and Vegetable Mould_. He shows
that each year worms bring up about 1/50th of an inch of soil, so that
if you laid a penny on the soil now and no one took it, in 50 years it
might be covered with an inch of soil. Pavements that were on the
surface when the Romans occupied Britain are now covered with a thick
layer of soil.

[Illustration: Fig. 25. Fresh soil turns milk bad, but baked soil does
not]

But besides these there are some living things too small to see, that
have only been found by careful experiments, but you can easily repeat
some of these {57} experiments yourselves. Div