he cup; what is the reason of this?

MRS. B.

These are bubbles of air which were partly attached to the vessel, and
partly diffused in the water itself; and they expand and rise in
consequence of the atmospheric pressure being removed.

CAROLINE.

See, Mrs. B.; the thermometer in the cup is sinking fast; it has already
descended to 40 degrees!

EMILY.

The water seems now and then violently agitated on the surface, as if it
was boiling; and yet the thermometer is descending fast!

MRS. B.

You may call it _boiling_, if you please, for this appearance is, as
well as boiling, owing to the rapid formation of vapour; but here, as
you have just observed, it takes place from the surface, for it is only
when heat is applied to the bottom of the vessel that the vapour is
formed there. --Now crystals of ice are actually shooting all over the
surface of the water.

CAROLINE.

How beautiful it is! The surface is now entirely frozen--but the
thermometer remains at 32 degrees.

MRS. B.

And so it will, conformably with our doctrine of latent heat, until the
whole of the water is frozen; but it will then again begin to descend
lower and lower, in consequence of the evaporation which goes on from
the surface of the ice.

EMILY.

This is a most interesting experiment; but it would be still more
striking if no sulphuric acid were required.

MRS. B.

I will show you a freezing instrument, contrived by Dr. Wollaston, upon
the same principle as Mr. Leslie's experiment, by which water may be
frozen by its own evaporation alone, without the assistance of sulphuric
acid.

This tube, which, as you see (PLATE V. fig. 2.), is terminated at each
extremity by a bulb, one of which is half full of water, is internally
perfectly exhausted of air; the consequence of this is, that the water
in the bulb is always much disposed to evaporate. This evaporation,
however, does not proceed sufficiently fast to freeze the water; but if
the empty ball be cooled by some artificial means, so as to condense
quickly the vapour which rises from the water, the process may be thus
so much promoted as to cause the water to freeze in the other ball. Dr.
Wollaston has called this instrument _Cryophorus_.

CAROLINE.

So that cold seems to perform here the same part which the sulphuric
acid acted in Mr. Leslie's experiment?

MRS. B.

Exactly so; but let us try the experiment.

EMILY.

How will you cool the instrument? You have neither ic