ace. Putty and clay likewise have very
little elasticity of form; when you change their shape, they
stay changed.

Jelly and steel and glass have a great deal of elasticity of
_form_. When you dent them or twist them or in any way change
their shape, they go right back to their first shape as soon
as they can.

When we imagined a man with an "elastic touch," we were
imagining a man who gave everything he touched perfect
elasticity of _form_. It is elasticity of _form_ that most
people mean when they talk about elasticity.

CHAPTER FOUR

HEAT

SECTION 15. _Heat makes things expand._

How does a thermometer work? What makes the mercury rise in
it?

Why does heat make things get larger?

When we look at objects through a microscope, they appear much larger
and in many cases we are able to see the smaller parts of which they
are made. If we had a microscope so powerful that it made a tiny
speck of dust look as big as a mountain (of course no such microscope
exists), and if we looked through this imaginary microscope at a piece
of iron, we should find to our surprise that the particles were not
standing still. The iron would probably look as if it were fairly
alive with millions of tiny specks moving back and forth, back and
forth, faster than the flutter of an insect's wings.

These tiny moving things are _molecules_. Everything in the world is
made of them. It seems strange that we should know this, since there
really are no microscopes nearly powerful enough to show the molecules
to us. Yet scientists know a great deal about them. They have devised
all sorts of elaborate experiments--very accurate ones--and have
tested the theories about molecules in many ways. They have said, for
instance, "Now, if this thing _is_ made of molecules, then it will
grow larger when we make the molecules move faster by heating it."
Then they heated it--in your next experiment you will see what
happened. This is only one of thousands of experiments they have
performed, measuring over and over again, with the greatest care,
exactly _how much_ an object expanded when it was heated a certain
amount; exactly how much heat was needed to change water to steam;
exactly how far a piece of steel of a certain size and shape could
bend without breaking; exactly how crystals form--and so on and so on.
And they have always found that everything acts as if it were made
of moving mole