5
years.

(In France and America a thousand millions is called a billion.)]

[Illustration: THE BROWNIAN MOVEMENT

A diagram, constructed from actual observations, showing the erratic
paths pursued by very fine particles suspended in a liquid, when
bombarded by the molecules of the liquid. This movement is called the
Brownian movement, and it furnishes a striking illustration of the truth
of the theory that the molecules of a body are in a state of continual
motion.]

The Energy of Atoms

And this is only the beginning of the wonders that were done with
"ordinary matter," quite apart from radium and its revelations, to which
we will come presently. Most people have heard of "atomic energy," and
the extraordinary things that might be accomplished if we could harness
this energy and turn it to human use. A deeper and more wonderful source
of this energy has been discovered in the last twenty years, but it is
well to realise that the atoms themselves have stupendous energy. The
atoms of matter are vibrating or gyrating with extraordinary vigour. The
piece of cold iron you hold in your hand, the bit of brick you pick up,
or the penny you take from your pocket is a colossal reservoir of
energy, since it consists of trillions of moving atoms. To realise the
total energy, of course, we should have to witness a transformation such
as we do in atoms of radio-active elements, about which we shall have
something to say presently.

If we put a grain of indigo in a glass of water, or a grain of musk in a
perfectly still room, we soon realise that molecules travel. Similarly,
the fact that gases spread until they fill every "empty" available space
shows definitely that they consist of small particles travelling at
great speed. The physicist brings his refined methods to bear on these
things, and he measures the energy and velocity of these infinitely
minute molecules. He tells us that molecules of oxygen, at the
temperature of melting ice, travel at the rate of about 500 yards a
second--more than a quarter of a mile a second. Molecules of hydrogen
travel at four times that speed, or three times the speed with which a
bullet leaves a rifle. Each molecule of the air, which seems so still in
the house on a summer's day, is really travelling faster than a rifle
bullet does at the beginning of its journey. It collides with another
molecule every twenty-thousandth of an inch of its journey. It is turned
from its course 5,000,000,