ed_ with
proper care, crystals of these substances may be caused to grow to a
great size.

The condition of perfect crystallization is, that the crystallizing
force shall act with deliberation. There should be no hurry in its
operations; but every molecule ought to be permitted, without
disturbance from its neighbours, to exercise its own rights. If the
crystallization be too sudden, the regularity disappears. Water may be
saturated with sulphate of soda, dissolved when the water is hot, and
afterwards permitted to cool. When cold the solution is
supersaturated; that is to say, more solid matter is contained in it
than corresponds to its temperature. Still the molecules show no sign
of building themselves together.

This is a very remarkable, though a very common fact. The molecules in
the centre of the liquid are so hampered by the action of their
neighbours that freedom to follow their own tendencies is denied to
them. Fix your mind's eye upon a molecule within the mass. It wishes
to unite with its neighbour to the right, but it wishes equally to
unite with its neighbour to the left; the one tendency neutralizes the
other and it unites with neither. But, if a crystal of sulphate of
soda be dropped into the solution, the molecular indecision ceases. On
the crystal the adjacent molecules will immediately precipitate
themselves; on these again others will be precipitated, and this act
of precipitation will continue from the top of the flask to the
bottom, until the solution has, as far as possible, assumed the solid
form. The crystals here produced are small, and confusedly arranged.
The process has been too hasty to admit of the pure and orderly action
of the crystallizing force. It typifies the state of a nation in which
natural and healthy change is resisted, until society becomes, as it
were, supersaturated with the desire for change, the change being then
effected through confusion and revolution.

Let me illustrate the action of the crystallizing force by two
examples of it: Nitre might be employed, but another well-known
substance enables me to make the experiment in a better form. The
substance is common sal-ammoniac, or chloride of ammonium, dissolved
in water. Cleansing perfectly a glass plate, the solution of the
chloride is poured over the glass, to which when the plate is set on
edge, a thin film of the liquid adheres. Warming the glass slightly,
evaporation is promoted, but by evaporation the water only