cold in melting.

EMILY.

That must be owing to the caloric which they absorb in passing from a
solid to a fluid form.

MRS. B.

That is, certainly, the most probable explanation.

_Sulphat of soda_, commonly called Glauber's salt, is another medicinal
salt, which is still more bitter than the preceding. We must prepare
some of these compounds, that you may observe the phenomena which take
place during their formation. We need only pour some sulphuric acid over
the soda which I have put into this glass.

CAROLINE.

What an amazing heat is disengaged! --I thought you said that cold was
produced by the melting of salts?

MRS. B.

But you must observe that we are now _making_, not _melting_ a salt.
Heat is disengaged during the formation of compound salts, and a faint
light is also emitted, which may sometimes be perceived in the dark.

EMILY.

And is this heat and light produced by the union of the opposite
electricities of the alkali and the acid?

MRS. B.

No doubt it is, if that theory be true.

CAROLINE.

The union of an acid and an alkali is then an actual combustion?

MRS. B.

Not precisely, though there is certainly much analogy in these
processes.

CAROLINE.

Will this sulphat of soda become solid?

MRS. B.

We have not, I suppose, mixed the acid and the alkali in the exact
proportions that are required for the formation of the salt, otherwise
the mixture would have been almost immediately changed to a solid mass;
but, in order to obtain it in crystals, as you see it in this bottle, it
would be necessary first to dilute it with water, and afterwards to
evaporate the water, during which operation the salt would gradually
crystallise.

CAROLINE.

But of what use is the addition of water, if it is afterwards to be
evaporated?

MRS. B.

When suspended in water, the acid and the alkali are more at liberty to
act on each other, their union is more complete, and the salt assumes
the regular form of crystals during the slow evaporation of its solvent.

Sulphat of soda liquefies by heat, and effloresces in the air.

EMILY.

Pray what is the meaning of the word _effloresces_? I do not recollect
your having mentioned it before.

MRS. B.

A salt is said to effloresce when it loses its water of crystallisation
on being exposed to the atmosphere, and is thus gradually converted into
a dry powder: you may observe that these crystals of sulphat of soda are
far from possessing the tra