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