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.

Sec. 2

The Wonder-World of Atoms

The exploration of this wonder-world of atoms and molecules by the
physicists and chemists of to-day is one of the most impressive triumphs
of modern science. Quite apart from radium and electrons and other
sensational discoveries of recent years, the study of ordinary matter is
hardly inferior, either in interest or audacity, to the work of the
astronomer. And there is the same foundation in both cases--marvellous
apparatus, and trains of mathematical reasoning that would have
astonished Euclid or Archimedes. Extraordinary, therefore, as are some
of the facts and figures we are now going to give in connection with the
minuteness of atoms and molecules