sfy your curiosity at present; for before we can attempt to
explain this apparent paradox, it is necessary to become acquainted with
the subject of LATENT HEAT: and that, I think, we must defer till our
next interview.

CAROLINE.

I believe, Mrs. B., that you are glad to put off the explanation; for it
must be a very difficult point to account for.

MRS. B.

I hope, however, that I shall do it to your complete satisfaction.

EMILY.

But before we part, give me leave to ask you one question. Would not
water, as well as ether, boil with less heat, if deprived of the
pressure of the atmosphere?

MRS. B.

Undoubtedly. You must always recollect that there are two forces to
overcome, in order to make a liquid boil or evaporate; the attraction of
aggregation, and the weight of the atmosphere. On the summit of a high
mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat
is required to make water boil, than in the plain, where the weight of
the atmosphere is greater.* Indeed if the weight of the atmosphere be
entirely removed by means of a good air-pump, and if water be placed in
the exhausted receiver, it will evaporate so fast, however cold it
maybe, as to give it the appearance of boiling from the surface. But
without the assistance of the air-pump, I can show you a very pretty
experiment, which proves the effect of the pressure of the atmosphere in
this respect.

Observe, that this Florence flask is about half full of water, and the
upper half of invisible vapour, the water being in the act of boiling.
--I take it from the lamp, and cork it carefully--the water, you see,
immediately ceases boiling. --I shall now dip the flask into a bason of
cold water.**

[Footnote *: On the top of Mount Blanc, water boiled when heated
only to 187 degrees, instead of 212 degrees.]

[Footnote **: The same effect may be produced by wrapping a cold
wet linen cloth round the upper part of the flask. In order to
show how much the water cools whilst it is boiling, a thermometer,
graduated on the tube itself, may be introduced into the bottle
through the cork.]

CAROLINE.

But look, Mrs. B., the hot water begins to boil again, although the cold
water must rob it more and more of its caloric! What can be the reason
of that?

MRS. B.

Let us examine its temperature. You see the thermometer immersed in it
remains stationary at 180 degrees, which is about 30 degrees below the
boiling