TREE CALORIC.

«'2

EMILY.

But before we part, give ine leave to ask vouone question. Would not water, as well as ether,boil with less heat, if the pressure of the atmos-phere were taken off?

MRS. n.

Undoubtedly. You must always recollect thatthere are two forces to overcome, in order tomake a liquid boil, or evaporate; the attractionof aggregation, and the weight of the atmos-phere. On the summit of a high mountain (asMr. De Saussure ascertained on Mount Blanc )much less heat is required to make water boilthan in the plain, where the weight of the at-mosphere is greater*.—But I can show you avery pretty experiment, which proves the effectof the pressure ol the atmosphere in this respect.

Observe, that this Florence flask is about halffull of water, and the upper half of invisiblevapour, the water being in the act of boiling.—Itake it from the lamp and cork it carefully—the 'I

water, you see, immediately ceases boiling.—Ishall now wrap a cold wet cloth round the upperpart of the flask f.

• On the top of Mount Blanc , water boiled when heated onlyto 187“.

+ Or the whole flask may be dipped into a bason of cold water.

In order to show how much the water cools whilst it is boiling, athermometer, graduated on the tube itself, may be introducedinto the bottle through the coik.