amount of heat we impart to the body, the
more rapid will be the molecular vibration, and
the wider the amplitude of the atomic oscillations.
It is the vibration of the molecules of a
solid which cause its expansion when heat is
applied to it. If the molecules, as is believed,
revolve round each other, the communication of
heat, by augmenting their centrifugal force,
may be supposed to push them more widely
asunder; exactly as a weight attached to a
spiral spring, if twirled in the air, tends to fly
away from the hand which holds it; and, as the
speed of revolution is augmented, the spring
dies more and more, and the distance
between the hand and the weight is increased.

When bodies are made to give forth any
sound, when the fiddle-string trembles beneath
the bow, when the bell vibrates at the stroke of
its clapper, their atoms move in cadence, like
the world in space. Between the imperceptible
molecules which move within limits of infinite
smallness, and the planetary globes which roll
in the firmament, there is no difference. The
harmony of the spheres is not an empty word.
A cause keeps the molecules of a body together;
the same cause prevents the heavenly bodies
from parting company. That cause is a force,
and it is the same force, in both cases; whether
it be called cohesion when it assembles atoms, or
gravitation when it groups stars in clusters.

Looking closer into the organisation of matter,
we shall find that force not only forms irregular
aggregations of molecules, but it works with
order and symmetry. Witness the phenomena
of crystallisation, to appreciate which, we need
go no further than the freezing of water and
the formation of snow. Professor Tyndall deftly
and delicately dissects a block of ice, by means
of a beam from his electric lamp: pulling the
crystal edifice to pieces by accurately reversing
the order of its architecture. Silently and
symmetrically the crystallising force had built
the atoms up; silently and symmetrically does
the electric beam take them down. Here we
have a star, and there a star; and as the
action continues, the ice appears to resolve itself
into stars, each one possessing six rays, each
one resembling a beautiful six-petalled flower.
By shifting the lens to and fro, new star-flowers
are brought into view; and as the action
continues, the edges of the petals become serrated,
spreading themselves out like fern-leaves.
Probably few are aware of the beauty latent in
a block of common ice. Only think, continues
our eloquent countryman, of lavish Nature
operating thus throughout the world! Every
atom of the solid ice which sheets the frozen
lakes of the north has been fixed according to
this law. Nature "lays her beams in music;"
and it is the function of science to purify our
organs, so as to enable us to hear the strain.
To many persons, a block of ice may seem of
no more interest and beauty than a block of
glass; but, in reality, it bears the same relation
to glass that an oratorio of Handel does to the
cries in a market-place. The ice is music, the
glass is noise; the ice is order, the glass is
confusion. In the glass, molecular forces constitute
an inextricably entangled skein; in the ice
they are woven into a symmetric web, of the
wonderful texture just described.

Snow-flakes are not less curious nor less
complicated in their structure. When the cold
is sharp enough to cause water to congeal, each
tiny droplet that hangs in the air gives birth to
a slim six-sided column terminated at each end
by a six-faced pyramid. These little crystals do
not remain isolated. During their descent they
cluster together, so forming star-shaped groups.
Sometimes six crystals only assemble round a
common centre—the simplest possible form of
star; but, in the majority of cases, the crystalline
associations are more numerous. On the
branches of the primary star, smaller crystals
are regularly disposed, and on these latter
smaller branchlets still. Thus the snowy star
grows more and more complicated, while every
additional ramification is made in obedience to
the one same law.

Our great English lecturer also tells us that
snow, perfectly formed, is not an irregular
aggregate of ice particles. In a calm atmosphere,
the aqueous atoms arrange themselves, so as to
form the most exquisite figures. The snow
crystals are built upon the same type as the
six-petalled flowers which show themselves
within a block of ice, when a beam of heat is
sent through it. The molecules arrange
themselves to form hexagonal stars. From a central
nucleus shoot six spiculæ, every two of which
are separated by an angle of sixty degrees.
From these central ribs, smaller spiculæ shoot
right and left with unerring fidelity, to the angle
of sixty degrees, and from these again other
smaller ones diverge at the same angle. These
frozen six-leaved blossoms constitute our mountain
snows. They load the Alpine heights,
where their frail architecture is soon destroyed
by the accidents of weather. Every winter
they fall, and every summer they disappear.
While they last, they assume the most wonderful
variety of form; their tracery is of the
finest frozen gauze; and, round about their
corners, other rosettes of smaller dimensions
often cling. Beauty is superposed upon beauty;
as if Nature, once committed to her task, took
delight in showing, even within the narrowest
limits, the wealth of her resources.

To behold this force in action, you have only
to watch the process of crystallisation under the
microscope—a most astounding spectacle
especially when seen with polarised light. Although
the atoms themselves are imperceptible, you
witness the rapid growth of their aggregation.
Invisible soldiers form into visible battalions,
arranging themselves regularly, as at the word of
command. The same troops, that is the same
solutions, never perform by mistake the
evolutions proper to others. Alum presents itself in
a mass with eight equal triangular faces; sea-
salt furnishes cubes; the prisms of rock crystal
are equally recognisable. Minerals have
a physiognomy, which reveals the constitution
of their bodies. Chemistry tells us that bodies
which are similar in form are fundamentally
similar; that is, if they affect the same