The "free path" of molecules, both in liquids and in gases, is so minute
as to be beyond the reach of the most powerful microscope. This free
path in liquids is a zigzag course, owing to the perpetual collisions
with other molecules. The molecular behavior of liquids differs from
that of gases only in what is called surface tension. Liquids have a
skin, a peculiar stress of the surface molecules; gases do not, but tend
to dissipate and fill all space. A drop of water remains intact till
vaporization sets in; then it too becomes more and more diffused.
When two substances combine chemically, more or less heat is evolved.
When the combination is effected slowly, as in an animal's body, heat is
slowly evolved. When the combustion is rapid, as in actual fire, heat is
rapidly evolved. The same phenomenon may reach the eye as light, and the
hand as heat, though different senses get two different impressions of
the same thing. So a mechanical disturbance may reach the ear as sound,
and be so interpreted, and reach the hand as motion in matter. In
combustion, the oxygen combines rapidly with the carbon, giving out heat
and light and carbon dioxide, but why it does so admits of no
explanation. Herein again is where life differs from fire; we can
describe combustion in terms of chemistry, but after we have described
life in the same terms something--and this something is the main
thing--remains untouched.
The facts of radio-activity alone demonstrate the truth of the atomic
theory. The beta rays, or emanations from radium, penetrating one foot
of solid iron are very convincing. And this may go on for hundreds of
years without any appreciable diminution of size or weight of the
radio-active substance. "A gram of such substance," says Sir Oliver
Lodge, "might lose a few thousand of atoms a second, and yet we could
not detect the loss if we continued to weigh it for a century." The
volatile essences of organic bodies which we detect in odors and
flavors, are not potent like the radium emanations. We can confine them
and control them, but we cannot control the rays of radio-active matter
any more than we can confine a spirit. We can separate the three
different kinds of rays--the alpha, the beta, and the gamma--by magnetic
devices, but we cannot cork them up and isolate them, as we can musk and
the attar of roses.
And these emanations are taking place more or less continuously all
about us and we know it not. In fact, we are at all times subjected to a
molecular bombardment of which we never dream; minute projectiles,
indivisible points of matter, are shot out at us in the form of
electrons from glowing metals, from lighted candles, and from other
noiseless and unsuspected batteries at a speed of tens of thousands of
miles a second, and we are none the wiser for it. Indeed, if we could
see or feel or be made aware of it, in what a different world we should
find ourselves! How many million-or billion-fold our sense of sight and
touch would have to be increased to bring this about! We live in a world
of collisions, disruptions, and hurtling missiles of which our senses
give us not the slightest evidence, and it is well that they do not.
There is a tremendous activity in the air we breathe, in the water we
drink, in the food we eat, and in the soil we walk upon, which, if
magnified till our senses could take it in, would probably drive us mad.
It is in this interior world of molecular activity, this world of
electric vibrations and oscillations, that the many transformations of
energy take place. This is the hiding-place of the lightning, of the
electrons which moulded together make the thunderbolt. What an
underworld of mystery and power it is! In it slumbers all the might and
menace of the storm, the power that rends the earth and shakes the
heavens. With the mind's eye one can see the indivisible atoms giving up
their electrons, see the invisible hosts, in numbers beyond the power of
mathematics to compute, being summoned and marshalled by some mysterious
commander and hurled in terrible fiery phalanxes across the battlefield
of the storm.
The physicist describes the atom and talks about it as if it were "a
tangible body which one could hold in his hand like a baseball." "An
atom," Sir Oliver Lodge says, "consists of a globular mass of positive
electricity with minute negative electrons embedded in it." He speaks of
the spherical form of the atom, and of its outer surface, of its centre,
and of its passing through other atoms, and of the electrons that
revolve around its centre as planets around a sun. The electron, one
hundred thousand times smaller than an atom, yet has surface, and that
surface is a dimpled and corrugated sheet--like the cover of a mattress.
What a flight of the scientific imagination is that!
The disproportion between the size of an atom and the size of an
electron is vastly greater than that between the sun and the earth.
Represent an atom, says Sir Oliver Lodge, by a church one hundred and
sixty feet long, eighty feet broad, and forty feet high; the electrons
are like gnats inside it. Yet on the electric theory of matter,
electrons are all of the atom there is; there is no church, but only the
gnats rushing about. We know of nothing so empty and hollow, so near a
vacuum, as matter in this conception of it. Indeed, in the new physics,
matter is only a hole in the ether. Hence the newspaper joke about the
bank sliding down and leaving the woodchuck-hole sticking out, looks
like pretty good physics. The electrons give matter its inertia, and
give it the force we call cohesion, give it its toughness, its strength,
and all its other properties. They make water wet, and the diamond hard.
They are the fountain-head of the immense stores of the inter-atomic
energy, which, if it could be tapped and controlled, would so easily do
all the work of the world. But this we cannot do. "We are no more
competent," says Professor Soddy, "to make use of these supplies of
atomic energy than a savage, ignorant of how to kindle a fire, could
make use of a steam-engine." The natural rate of flow of this energy
from its atomic sources we get as heat, and it suffices to keep life
going upon this planet. It is the source of all the activity we see upon
the globe. Its results, in the geologic ages, are stored up for us in
coal and oil and natural gas, and, in our day, are available in the
winds, the tides, and the waterfalls, and in electricity.