Lessons on Soil | Page 8

this because clay is lighter than water? Probably
not, because a lump of clay seems very heavy. Further, if we put a
small ball of clay into water it at once sinks to the bottom. Only when
we rub the clay between our fingers or work it with a stick--in other
words, when we break the ball into very tiny pieces--can we get it to
float again. We therefore conclude that the clay floated in our jars (p. 6)
for so long not because it was lighter than water, but because the pieces
were so small.
Clay is exceedingly useful because of its stickiness. Dig up some clay,
if there is any in your garden, or procure some from a brick works. You

can mould it into any shape you like, and the purer the clay the {10}
better it acts. Enormous quantities of clay are used for making bricks.
Make some model bricks about an inch long and half an inch in width
and depth, also make a small basin of about the same size, then set
them aside for a week in a warm, dry place. They still keep their shape;
even if a crack has appeared the pieces stick together and do not
crumble to a powder.
If you now measure with a ruler any of the bricks that have not cracked,
you will find that they have shrunk a little and are no longer quite an
inch long. This fact is well known to brickmakers; the moulds in which
they make the bricks are larger than the brick is wanted to be. But what
would happen if instead of a piece of clay one inch long you had a
whole field of clay? Would that shrink also, and, if so, what would the
field look like? We can answer this question in two ways; we may
make a model of a field and let it dry, and we can pay a visit to a clay
meadow after some hot, dry weather in summer. The model can be
made by kneading clay up under water and then rolling it out on some
cardboard or wood as if it were a piece of pastry. Cut it into a square
and draw lines on the cardboard right at the edges of the clay. Then put
it into a dry warm place and leave for some days. Fig. 4 is a picture of
such a model after a week's drying. The clay has shrunk away from the
marks, but it has also shrunk all over and has cracked. If you get an
opportunity of walking over a clay field during a dry summer, you will
find similar but much larger cracks, some of which may be two or three
inches wide, or even more. Sometimes the cracking is so bad that the
roots of plants or of trees are torn by it, and even buildings, in some
instances, have suffered through their foundations shrinking away. {11}
We can now understand why some of our model bricks cracked. The
cracks were caused by the shrinkage just as happens with our model
field. As soon as the clay becomes wet it swells again. A very pretty
experiment can be made to show this. Fill a glass tube or an egg-cup
with dry powdered clay, scrape the surface level with a ruler, and then
stand in a glass jar full of rain water so that the whole is completely
covered. After a short time the clay begins to swell and forces its way
out of the egg-cup as shown in Fig. 5, falling over the side and making
quite a little shower. In exactly the same way the ground swells after

heavy rain and rises a little, then it falls again and cracks when it
becomes dry. Darwin records some careful measurements in a book
called Earthworms and Vegetable Mould--"a large flat stone laid on the
surface of a field sank 3.33 millimetres[1] whilst the weather was dry
between May 9th and June 13th, {12} and rose 1.91 millimetres
between September 7th and 19th of the same year, much rain having
fallen during the latter part of this time. During frosts and thaws the
movements were twice as great."
[Illustration: Fig. 4. Clay was plastered over a square piece of board
and completely covered it. After drying for a week the clay had shrunk
and cracked]
You must have found out by now how very slippery clay becomes as
soon as it is wet enough. It is not easy to walk over a clay field in wet
weather, and if the clay forms part of the slope of a hill it may be so
slippery that it becomes dangerous. Sometimes after very heavy rains
soil resting on clay on the side of a hill has