se 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 thei