This I understand now very well; but if the water freezes in consequence of yielding its caloric to the ether, the equilibrium of heat must, in this case, be totally destroyed. Yet you have told us, that the exchange of caloric between two bodies of equal temperature, was always equal; how, then, is it that the water, which was originally of the same temperature as the ether, gives out caloric to it, till the water is frozen, and the ether made to boil? MRS. B. I suspected that you would make these objections; and, in order to remove them, I enclosed two thermometers in the air-pump; one which stands in the glass of water, the other in the phial of ether; and you may see that the equilibrium of temperature is not destroyed; for as the thermometer descends in the water, that in the ether sinks in the same manner; so that both thermometers indicate the same temperature, though one of them is in a boiling, the other in a freezing liquid. EMILY. The ether, then, becomes colder as it boils? This is so contrary to common experience, that I confess it astonishes me exceedingly. CAROLINE. It is, indeed, a most extraordinary circumstance. But pray, how do you account for it? MRS. B. I cannot satisfy 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.†