But if we had some magical glass by means of which we could see into the
structure of material things, we should not see the atoms put evenly
together as bricks are in a wall. As a rule, two or more atoms first
come together to form a larger particle, which we call a "molecule."
Single atoms do not, as a rule, exist apart from other atoms; if a
molecule is broken up, the individual atoms seek to unite with other
atoms of another kind or amongst themselves. For example, three atoms of
oxygen form what we call ozone; two atoms of hydrogen uniting with one
atom of oxygen form water. It is molecules that form the mass of matter;
a molecule, as it has been expressed, is a little building of which
atoms are the bricks.
In this way we get a useful first view of the material things we handle.
In a liquid the molecules of the liquid cling together loosely. They
remain together as a body, but they roll over and away from each other.
There is "cohesion" between them, but it is less powerful than in a
solid. Put some water in a kettle over the lighted gas, and presently
the tiny molecules of water will rush through the spout in a cloud of
steam and scatter over the kitchen. The heat has broken their bond of
association and turned the water into something like a gas; though we
know that the particles will come together again, as they cool, and form
once more drops of water.
In a gas the molecules have full individual liberty. They are in a
state of violent movement, and they form no union with each other. If we
want to force them to enter into the loose sort of association which
molecules have in a liquid, we have to slow down their individual
movements by applying severe cold. That is how a modern man of science
liquefies gases. No power that we have will liquefy air at its ordinary
temperature. In _very_ severe cold, on the other hand, the air will
spontaneously become liquid. Some day, when the fires of the sun have
sunk very low, the temperature of the earth will be less than -200° C.:
that is to say, more than two hundred degrees Centigrade below
freezing-point. It will sink to the temperature of the moon. Our
atmosphere will then be an ocean of liquid air, 35 feet deep, lying upon
the solidly frozen masses of our water-oceans.
In a solid the molecules cling firmly to each other. We need a force
equal to twenty-five tons to tear asunder the molecules in a bar of iron
an inch thick. Yet the structure is not "solid" in the popular sense of
the word. If you put a piece of solid gold in a little pool of mercury,
the gold will take in the mercury _between_ its molecules, as if it were
porous like a sponge. The hardest solid is more like a lattice-work than
what we usually mean by "solid"; though the molecules are not fixed,
like the bars of a lattice-work, but are in violent motion; they vibrate
about equilibrium positions. If we could see right into the heart of a
bit of the hardest steel, we should see billions of separate molecules,
at some distance from each other, all moving rapidly to and fro.
This molecular movement can, in a measure, be made visible. It was
noticed by a microscopist named Brown that, in a solution containing
very fine suspended particles, the particles were in constant movement.
Under a powerful microscope these particles are seen to be violently
agitated; they are each independently darting hither and thither
somewhat like a lot of billiard balls on a billiard table, colliding and
bounding about in all directions. Thousands of times a second these
encounters occur, and this lively commotion is always going on, this
incessant colliding of one molecule with another is the normal
condition of affairs; not one of them is at rest. The reason for this
has been worked out, and it is now known that these particles move about
because they are being incessantly bombarded by the molecules of the
liquid. The molecules cannot, of course, be seen, but the fact of their
incessant movement is revealed to the eye by the behaviour of the
visible suspended particles. This incessant movement in the world of
molecules is called the Brownian movement, and is a striking proof of
the reality of molecular motions.
§ 2