t escape. I have put a thermometer in the glass of water that
is to be frozen, in order that you may see how it cools.

CAROLINE.

The thermometer descends, but the heat which the water is now losing, is
its _free_, not its _latent_ heat.

MRS. B.

Certainly; it does not part with its latent heat till it changes its
state and is converted into ice.

EMILY.

But here is a very extraordinary circumstance! The thermometer is fallen
below the freezing point, and yet the water is not frozen.

MRS. B.

That is always the case previous to the freezing of water when it is in
a state of rest. Now it begins to congeal, and you may observe that the
thermometer again rises to the freezing point.

CAROLINE.

It appears to me very strange that the thermometer should rise the very
moment that the water freezes; for it seems to imply that the water was
colder before it froze than when in the act of freezing.

MRS. B.

It is so; and after our long dissertation on this circumstance, I did
not think it would appear so surprising to you. Reflect a little, and I
think you will discover the reason of it.

CAROLINE.

It must be, no doubt, the extrications of latent heat, at the instant
the water freezes, that raises the temperature.

MRS. B.

Certainly; and if you now examine the thermometer, you will find that
its rise was but temporary, and lasted only during the disengagement of
the latent heat--now that all the water is frozen it falls again, and
will continue to fall till the ice and mixture are of an equal
temperature.

EMILY.

And can you show us any experiments in which liquids, by being mixed,
become solid, and disengage latent heat?

MRS. B.

I could show you several; but you are not yet sufficiently advanced to
understand them well. I shall, however, try one, which will afford you a
striking instance of the fact. The fluid which you see in this phial
consists of a quantity of a certain salt called _muriat of lime_,
dissolved in water. Now, if I pour into it a few drops of this other
fluid, called _sulphuric acid_, the whole, or very nearly the whole,
will be instantaneously converted into a solid mass.

EMILY.

How white it turns! I feel the latent heat escaping, for the bottle is
warm, and the fluid is changed to a solid white substance like chalk!

CAROLINE.

This is, indeed, the most curious experiment we have seen yet. But pray
what is that white vapour that ascends from the mixture?

MRS. B