ure of the air be as high as
60 degrees. The water is put into shallow earthen trays, so as to expose
an extensive surface to the process of evaporation, and in the morning,
the water is found covered with a thin cake of ice, which is collected
in sufficient quantity to be used for purposes of luxury.

CAROLINE.

How delicious it must be to drink liquids so cold in those tropical
climates! But, Mrs. B., could we not try that experiment?

MRS. B.

If we were in the country, I have no doubt but that we should be able to
freeze water, by the same means, and under similar circumstances. But we
can do it immediately, upon a small scale, in this very room, in which
the thermometer stands at 70 degrees. For this purpose we need only
place some water in a little cup under the receiver of the air-pump
(PLATE V. fig. 1.), and exhaust the air from it. What will be the
consequence, Caroline?

[Illustration: Plate V. Vol. I. page 138.

Fig. 1.
The air-pump & receiver for Mr. Leslie's experiment.
C a saucer with sulphuric Acid.
B a glass or earthen cup containing Water.
D a stand for the cup with its legs made of Glass.
A a Thermometer.

Fig. 2. Dr. Wollaston's Cryophorus.

Fig. 5. Dr. Marcet's mode of using the Cryophorus.

Fig. 3. & 4. the different parts of Fig. 5. seen separate.]

CAROLINE.

Of course the water will evaporate more quickly, since there will no
longer be any atmospheric pressure on its surface: but will this be
sufficient to make the water freeze?

MRS. B.

Probably not, because the vapour will not be carried off fast enough;
but this will be accomplished without difficulty if we introduce into
the receiver (fig. 1.), in a saucer, or other large shallow vessel, some
strong sulphuric acid, a substance which has a great attraction for
water, whether in the form of vapour, or in the liquid state. This
attraction is such that the acid will instantly absorb the moisture as
it rises from the water, so as to make room for the formation of fresh
vapour; this will of course hasten the process, and the cold produced
from the rapid evaporation of the water, will, in a few minutes, be
sufficient to freeze its surface.* We shall now exhaust the air from the
receiver.

[Footnote *: This experiment was first devised by Mr. Leslie, and
has since been modified in a variety of forms.]

EMILY.

Thousands of small bubbles already rise through the water from the
internal surface of t