performed. Fetch it, and we shall
examine it.

EMILY.

What a variety of processes the decomposition of water, by a metal and
an acid, implies; 1st, the decomposition of the water; 2dly, the
oxydation of the metal; and 3dly, the formation of a compound salt.

CAROLINE.

Here it is, Mrs. B. --What beautiful green crystals! But we do not
perceive any crystals in the solution of copper in nitrous acid?

MRS. B.

Because the salt is now suspended in the water which the nitrous acid
contains, and will remain so till it is deposited in consequence of rest
and cooling.

EMILY.

I am surprised that a body so opake as iron can be converted into such
transparent crystals.

MRS. B.

It is the union with the acid that produces the transparency; for if the
pure metal were melted, and afterwards permitted to cool and
crystallise, it would be found just as opake as before.

EMILY.

I do not understand the exact meaning of _crystallisation_?

MRS. B.

You recollect that when a solid body is dissolved either by water or
caloric it is not decomposed; but that its integrant parts are only
suspended in the solvent. When the solution is made in water, the
integrant particles of the body will, on the water being evaporated,
again unite into a solid mass by the force of their mutual attraction.
But when the body is dissolved by caloric alone, nothing more is
necessary, in order to make its particles reunite, than to reduce its
temperature. And, in general, if the solvent, whether water or caloric,
be slowly separated by evaporation or by cooling, and care taken that
the particles be not agitated during their reunion, they will arrange
themselves in regular masses, each individual substance assuming a
peculiar form or arrangement; and this is what is called
crystallisation.

EMILY.

Crystallisation, therefore, is simply the reunion of the particles of a
solid body that has been dissolved in a fluid.

MRS. B.

That is a very good definition of it. But I must not forget to observe,
that _heat_ and _water_ may unite their solvent powers; and, in this
case, crystallisation may be hastened by cooling, as well as by
evaporating the liquid.

CAROLINE.

But if the body dissolved is of a volatile nature, will it not evaporate
with the fluid?

MRS. B.

A crystallised body held in solution only by water is scarcely ever so
volatile as the fluid itself, and care must be taken to manage the heat
so that it may be suffici