or aggregation into a mass, are found to
arrange themselves into regular and symmetrical
forms, bounded by flat sides and angles, and
that each substance is found to assume always
its own characteristic shape, from which
it never varies,—whether this be a pyramid,
a cube, or a prism. The number of known
crystalline forms is much smaller than the
number of crystallisable substances; and it
follows that crystals of various kinds may
possess the same form; but no substance is
ever found to assume a form foreign to its
usual habit. Some, it is true, assume more
than one form; but even these keep strictly
within their proper range, so that the shape
of a crystal affords some evidence of its
nature. The peculiar shape of the crystals of
white arsenic furnishes one means of
identifying this substance in medico-legal
inquiries; and the same principle rules under
a very great variety of circumstances.

All that is absolutely necessary to the
process of crystallisation is to reduce the
substance treated, to the fluid or gaseous state,
either by heat alone, or by heat with this
assistance of a solvent; and then to cool it
so gradually, that its loosened or separated
particles have time to select, as it were, the
shape which they will assume. If a piece of
cast metal be broken, the newly-exposed
surface will always present a crystalline
appearance, the result of the re-arrangement of its
particles while hardening from the state of
fusion. Crystals may also be obtained from
melted sulphur by a method of treatment
which it is needless to describe; and camphor,
or white arsenic, if heated in a glass
tube, will be first volatilised, and then
deposited in a ring of crystals higher up, at the
first point where the temperature is
sufficiently low. All these are instances of
crystallisation without a solvent, and are
adduced as showing the essential nature of
the process. In obtaining crystals of
substances that are commonly used in that form,
it is the practice to employ solvents; because
the resistance of the liquid medium renders
the process more gradual, affords opportunities
of modifying it by art, and enables the
operator to obtain large and perfect results.
The crystalline form is to some extent a
guarantee of purity, or, at least, of the
absence of adulteration: hence, both in
medicine and the arts, many salts are subjected
to tedious processes for no other purpose than
to assume this form.

When some substance readily soluble and
crystallisable, as saltpetre, alum, Glauber's
salt, or Epsom salt, is put into water at the
temperature of the atmosphere, the mixture
immediately becomes colder. It requires
heat, in order to loosen and separate the
integral particles of the salt, so that the
water may dissolve them; and thus the salt
absorbs and removes heat from the water
into itself, and the water removes heat from
all surrounding things—from the air, from
the containing vessel, from the hand, or the
thermometer by which this vessel may be
touched. After a time, this process comes to
an end, either by the salt being all dissolved,
or by the water having dissolved as much as
it is able to sustain; and then the mixture is
gradually raised to the temperature of the
air around it. If it be now carried to a
colder place, some of the heat necessary to
keep the particles of the salt asunder and
in solution is withdrawn; and these particles,
in quantity proportionate to the fall of
temperature, cohere into crystals, and attach
themselves to the sides or bottom of the
containing vessel, or to any projecting point that
may offer itself. If the temperature were
then raised, they would be re-dissolved; and
if it were then again lowered, they would
once more be deposited. In practice, it is
found most expedient to make hot solutions,
from which crystallisation takes place as they
cool—an arrangement which supersedes the
necessity of any contrivances for lowering
the temperature. In a general way, it is
important to the process of crystallisation that
the liquid should not be shaken or disturbed,
but some slight motion between its particles
seems necessary in order to initiate the
process, which does not commence at all in a
state of absolute repose. A saturated hot
solution of Glauber's salt, if allowed to cool
in perfect stillness, will remain liquid as long
as the stillness is preserved, but the slightest
movement or tremor—even a wave of the
hand through the air in its vicinity—will
instantly transform the solution into a solid
mass, some of the water entering into the
composition of the crystals, and some being
retained by interstices in their structure. In
the same manner, water may be cooled to a
very low temperature without change; and
then, upon the slightest disturbance, will
freeze (i.e., crystallise) throughout. But,
when the process is once set on foot, any
agitation diminishes the size of the crystals,
a similar effect being produced if the solution
be permitted to cool too rapidly. Hence, in
pursuing crystallisation as an art, there are
many minutiæ that require attention.

That familar cottage ornament, the alum-
basket, and the equally familiar sugar-candy,
illustrate the predilection of crystals for some
point of attachment nearer than the sides of
the vessel, and the certainty with which the
process is carried on around any such nucleus
that is provided. It is well known that
strings are strained through the syrup to
obtain the candy, and that the little basket is
suspended in the alum solution. Then the
tiny and invisible particles are gradually
drawn together towards the foundation thus
afforded, and presently little glittering specks
may be discerned entangled among the fibres
of the threads, or studding the network of
the basket. If the matter be well managed,
these specks increase steadily in size, by the
regular addition of fresh atoms to every part;