Lessons on Soil | Page 9

begun to slide downwards
and moves some distance before it stops. Fortunately these land slips as
they are called, are not common in England, but they do occur. Fig. 6
shows one in the Isle of Wight, and another is described by Gilbert
White in The Natural History of Selborne.
[Illustration: Fig. 5. Clay swelling up when placed in water and
overflowing from the egg-cup into which it was put]
[Illustration: Fig. 6. Landslip in the Isle of Wight]
Another thing that you will have noticed is that anything made of clay
holds water. A simple way of testing this is to put a round piece of tin
perforated {14} with holes into a funnel, press some clay on to it and
on to the sides of the funnel (Fig. 7), and then pour on rain water. The
water does not run through. Pools of water may lie like this on a clay
field for a very long time in winter before they disappear, as you will
know very well if you live in a clay country. So when a lake or a
reservoir is being made it sometimes happens that the sides are lined
with clay to keep the water in.

[Illustration: Fig. 7. A thin layer of clay a entirely prevents the water
running through]
If water cannot get through can air? This is very easily discovered: plug
a glass tube with clay and see if you can draw or blow air through. You
cannot. Clay can be used like putty to stop up holes or cracks, and so
long as it keeps moist it will neither let air nor water {15} through.
Take two bottles like those in Fig. 8, stop up the bottom tubes, and fill
with water. Then put a funnel through each cork and fit the cork in
tightly, covering with clay if there is any sign of a leak. Put a perforated
tin disk into each funnel, cover one well with clay and the other with
sand. Open the bottom tubes. No water runs out from the first bottle
because no air can leak in through the clay, but it runs out very quickly
from the second because the sand lets air through. These properties of
clay and sand are very important for plants. Sow some seeds in a little
jar {16} full of clay kept moist to prevent it cracking, and at the same
time sow a few in some moist sand. The seeds soon germinate in the
sand but not in the clay. It is known that seeds will not germinate
unless they have air and water and are warm enough. They had water in
both jars, and they were in both cases warm, but they got no air through
the clay and therefore could not sprout. Pure clay would not be good
for plants to grow in. Air came through the sand, however, and gave the
seeds all they wanted for germination.
[Illustration: Fig. 8. Sand allows air to pass through it, and so water
runs out of the bottle. Clay does not let air pass, and the water is
therefore kept in, even though the tube is open.]
This also explains something else that you may have noticed. If you
tried baking one of your model bricks in the fire you probably found
that the brick exploded and shattered to pieces: the water still left in the
brick changed to steam when it was heated, but the steam could not
escape through the clay, and so it burst the clay. In a brick works the
heat is very gradually applied and the steam only slowly forms, so that
it has time to leak away, then when it has all gone the brick can be
heated strongly. You should try this with one of your model bricks;
leave it in a hot place near the stove or on the radiator for a week or

more and then see if you can bake it without mishap.
Let us now compare a piece of clay with a brick. The differences are so
great that you would hardly think the brick could have been made from
clay. The brick is neither soft nor sticky, and it has not the smooth
surface of a piece of clay, but is full of little holes or pores, which look
as if they were formed in letting the steam out. A brick lets air through;
some air gets into our houses through the bricks even when the
windows are shut. Water will get through bricks more easily than it
does through clay. After heavy rain you {17} can often find that water
has soaked through a brick wall and made the wall paper quite damp. A
pretty experiment can be made with the piece of apparatus shown in
Fig.