In imagining the ultimate composition of a
solid body, we have to reconcile two apparently
contradictory conditions.  It is an assemblage
of atoms which do not touch each other—for we
are obliged to admit intermolecular spaces—and
yet those atoms are held together in clusters by
so strong a force of cohesion as to give to the
whole the qualities of a solid.  This would be
the case even with a solid undergoing no change
of size or internal constitution . But solids do
change, under pressure, impact, heat, and cold.
Their constituent atoms are, consequently, not
at rest.  Mr. Grove tells us: "Of absolute rest
Nature gives us no evidence.  All matter, as
far as we can ascertain, is ever in movement,
not merely in masses, as with the planetary
spheres, but also molecularly, or throughout its
most intimate structure.  Thus, every alteration
of temperature produces a molecular change
throughout the whole substance heated or
cooled.  Slow chemical or electrical actions,
actions of light or invisible radiant forces, are
always at play; so that, as a fact, we cannot
predicate of any portion of matter that it is
absolutely at rest."

The atoms, therefore, of which solid bodies
consist, are supposed to vibrate, to oscillate, or,
better, to revolve, like the planets, in more or
less eccentric orbits.  Suppose a solid body to
be represented by a swarm of gnats dancing in
the sunshine.  Each gnat, or atom, dances up
and down, at a certain distance from each other
gnat, within a given limited space.  The path of
the dance is not a mere straight line, but a
vertical oval—a true orbit.  Suppose, then, that in
consequence of greater sun heat, the gnats
become more active, and extend each its respective
sweep of flight. The swarm, or solid body,
as a whole, expands. If, from a chill or the
shadow of a cloud, the insect's individual range
is less extensive, the crowd of gnats is
necessarily denser, and the swarm, in its integrity,
contracts.

Tyndall takes for his illustration a bullet
revolving at the end of a spiral spring.  He had
spoken of the vibration of the molecules of a
solid as causing its expansion; but he remarks
that, by some, the molecules have been thought
to revolve round each other; and the communication
of heat, by augmenting their centrifugal
force, was supposed to push them more widely
asunder.  So he twirls the weight, at the end of
the spring, in the air.  It tends to fly away;
the spring stretches to a certain extent; and, as
the speed of revolution is augmented, the spring
stretches still more, the distance between his
hand and the weight being thus increased.  The
spring rudely figures the force of cohesion, while
the ball represents an atom under the influence
of heat.

The intellect, he truly says, knows no difference
between great and small.  It is just as
easy, as an intellectual act, to picture a vibrating
or revolving atom as to picture a vibrating or
revolving cannon-ball.  These motions, however,
are executed within limits too minute, and
the moving particles are too small, to be visible.
Here the imagination must help us.  In the
case of solid bodies, you must conceive a power
of vibration, within certain limits, to be possessed
by the molecules.  You must suppose them
oscillating to and fro; the greater the amount
of heat we impart to the body, the more rapid
will be the molecular vibration, and the wider
the amplitude of atomic oscillations.

It is held that all matter differs only in the
grouping of its elements—in the juxta-position
of its molecules.  That juxta-position depends on
the temperature, and the speed with which
changes of temperature have taken place.  The
mode and manner of those changes are so many
causes of the transformation of matter—so many
origins of divers substances.  It is maintained
that, in the actual state of science, bodies differ
only by the clustering of their atoms, exactly as
the constellations of the sky differ through the
arrangement of their stars.

Take a bird's-eye view, from the car of a
balloon, of four or five towns, at a considerable
altitude.  They will differ but very slightly in
aspect; they are simply towns.  From a point
of view nearer to the earth, their distinctive
characters will be visible; showing themselves
in the disposition of the houses, the topography
of the streets, and the distribution of the public
walks.  Such is the case with a mineral or any
other substance whatever.  Accordingly, as
natural forces have laid out, on this or that
plan, the walks, streets, and houses, of our little
molecular cities, they strike you with a different
impression.  The one depends on the will of
the architect, the other on the action of the
predominant force.

Wax, for instance, is cited by our great lecturer
as expanding, in passing from the solid to
the liquid state. To assume the liquid form, its
particles must be pushed more widely apart—
a certain play between them being necessary to
the condition of liquidity.  Ice, on the contrary,
on liquifying, contracts.  In the arrangement of
its atoms to form a solid, more room is required
than those atoms need in the neighbouring liquid
state.  No doubt this is due to crystalline
arrangement. The attracting poles of the molecules
are so situated, that, when the crystallising force
comes into play, the molecules unite, so as to
leave larger interatomic spaces in the mass.
We may suppose them to attach themselves by
their corners; and, in turning corner to corner,
to cause a recession of the atomic centres.  At
all events, their centres retreat from each other
when solidification sets in.

The atoms of bodies must be regarded as all
but infinitely small; the necessary consequence
of which, is, that they must be all but infinitely
numerous. A learned Frenchman, Monsieur
A. Gaudin, calculator at the Bureau des
Longitudes, has lately estimated, by a very
ingenious process, the distances which separate
molecules and their component atoms, and their
number.  The result he obtains is, that, if you
set about counting the atoms contained in a
little cube of solid matter two millimetres high
—that is, about the size of a pin's-head—and
that you counted a billion of them per second, it
would take you about two hundred and fifty