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. 9: bore in a brick a hole about an inch deep
and a quarter of an inch wide, put into the hole the piece of bent
glass tubing, and fix it in with some clay or putty, then pour some
water blackened with ink into the tube, marking its position with a
label. Stand the brick in a vessel so full of water that the brick is
entirely covered. Water soaks into the brick and presses the air out:
the air tries to escape through the tube and forces up the black liquid.

[Illustration: Fig. 9. A brick standing in water. The air in the
brick is driven inwards by the water and forces the liquid up the tube
in order to escape]

One more experiment may be tried. Can a brick be changed back into
clay? Grind up the brick and it forms a gritty powder. Moisten it,
work it with your fingers how you please, but it still remains a gritty
powder and never takes on the greasy, sticky feeling of {18} pure clay.
Indeed no one has ever succeeded in making clay out of bricks. All
these experiments show that clay is completely altered when it is
burnt. We also found that soil is completely altered by burning, and
if you look back at your notes you will see that the changes are very
much alike, so much so that we can safely put down some of the changes
in the burnt soil--the red colour, the hard grittiness, and the absence
of stickiness--to the clay. Let us now examine a piece of dry, but
unburnt, clay. It is very hard and does not crumble, it is neither
sticky nor slippery. Directly, however, we add some water it changes
back to w