particles to fall or otherwise settle themselves
in an uniform direction, the cube would be
the only form capable of reproduction in this
manner, so the hypothesis of atomic polarity,
analogous to the polarity of the magnetic
needle, has been added to the hypothesis of
atomic form, and made to explain why a
countless multitude of prisms or pyramids—
each invisible by reason of its minuteness—
should fall in the same manner and into the
fitting place, so as to build up one prismatic
or pyramidal crystal, instead of an incoherent
heap, or a lump, shapeless and full of
interstices. Such is the nearest approach
philosophers have made, as yet, to an
explanation of the causes of crystalline forms; as
shown either in the miniature rockwork
which is thrown down daily, in hundreds of
laboratories, from solutions of almost every
kind, or in the many other ways with which
science is familiar.

For, the process of crystallisation, although
most commonly seen as it takes place in
solutions, is by no means limited to the conditions
which they afford; it may be observed
wherever, in consequence of a loss of heat,
any inorganic substance passes from a fluid
into a solid form. This change appears,
iudeed, to be the only essential to the formation
of a crystal; and it may be shown to
have occurred in every case when a crystal
has been formed.

It is a property common to almost all
substances, that they expand, or increase in
size, by the application of heat,—a result,
that is explained by supposing them to
consist of ultimate particles or atoms, which,
when heated, tend to move apart by a mutual
repulsion, and to enlarge the bulk of the
whole mass, by a gradual loosening and
separation of its texture. If the heat be carried
to a certain extent, the substance heated will
have its particles driven altogether away
from each other, and diffused in the surrounding
medium,—whatever that may be,—whether
air, water, or spirit: a result that is
called volatilisation in the first case, and
solution in the others. Take, for instance, a
piece of camphor, and place it in spirit of
wine. Whenever the spirit is warmer than
the camphor, minute particles of the latter
will be taken up, and diffused invisibly
throughout the former—being said to be
dissolved in it. If the camphor be simply
placed under a bell-glass, whenever the
surrounding air is warmer than the lump,
minute particles will be taken up as before,
and diffused invisibly throughout this air:
being readily detected, although invisible,
by the more delicate senses of taste
and smell. This result is commonly called;
volatilisation, or diffusion in the form of
vapours, rather than solution; but it is, in
fact, a true solution,—a solution in air instead
of in spirit. And it will be found universally,
and may be shown by a hundred familiar
examples in daily life, that the compactness
of everything depends upon its temperature;
diminishing as heat is added, increasing as
heat is withdrawn. Thus, strong soup—a
jelly unless artificially heated—liquifies when
placed over the fire; in the same way,
metals and other substances are melted by
heat: that is, are reduced to a yielding
and fluent state. Thus, ice is converted
into water, and water into steam,
which is dissolved in the surrounding air.
Materials that are flexible and elastic,
at ordinary temperatures, become hard, and
even brittle, during frost: such things as
caoutchouc, gutta-percha, and sealing-wax,
being suitable examples. The rule is
unlimited in its application; although more
evident in some examples than in others.

There are, however, many substances
which, when volatilised in air, or dissolved
in any liquid, are found to undergo changes
of a chemical nature, and no longer to exist
in their original form. Thus charcoal, heated
in the air, gives off minute atoms which
combine immediately with the oxygen around
them,—forming a noxious gas,—and cease to
be charcoal by virtue of the combination.
When iron is dissolved in a diluted acid, the
resulting liquid does not contain finely
divided iron, but finely divided salt of that
metal, produced by its union with the acid
employed. And so of many other instances,
But there is a numerous class of bodies, both
simple and compound, that may be either
dissolved or volatilised without suffering any
other change: their atoms, although
separated and scattered, never losing their
individuality and identity. Thus camphor—
whether a lump in the hand or floating in the
air or dissolved in spirit—is always the same
substance.

It is with this latter class only that we
have now to deal, that is to say, with
substances whose atoms, when scattered by heat
throughout some dissolving or suspending
medium, undergo no change beyond the mere
dispersion, and, in the inorganic kingdom of
nature, act the part of Jews among mankind,
For such as these, when once the force that
separated them is removed, have a tendency
to draw lovingly together, and to reconstitute
the original solidity of the mass from
which they sprung. Their union is hindered
by the medium around them, in exact proportion
to its density and resisting power. Thus,
atoms which are dispersed in air unite
more rapidly than those dissolved in spirit;
and those dissolved in spirit more readily
than those dissolved in water. But this
resistance of the medium is usually overcome,
unless the particles be very few and very
remote; and, coincidently with a decline of
temperature, the substance that was
volatilised or dissolved is again recovered. So
much might, perhaps, have been expected.
But what—prior to experience—could
not have been anticipated, is that the
scattered atoms, not content with mere re-union,