vacuum, becomes only a partial vacuum, since the space contains a certain amount of water vapor. After a time, no further vapor will be given off, and there will be a certain definite pressure in the vessel. If, however, the vapor given off by the water is continuously removed, so as constantly to maintain a vacuum, the water will continue to give off vapor indefinitely, or until so much heat has been abstracted from the water that it will freeze. That it is possible to freeze water by its own evaporation may be shown by experiment.
48. A flat dish, Fig. 7, containing a small quantity of water, is supported on a tripod in such a way that little or no heat can be carried to it. Beneath this dish is placed another containing sulphuric acid, which has a strong affinity for water vapor. Both dishes are placed under a glass vessel a
and the air is removed from the inside of this vessel through
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the hole b in the plate c, by means of an air pump not shown in the illustration. Although the water may be at the temperature of the outside air, it will presently begin to boil. The vapor given off will be, to a large extent, absorbed by the acid, and if the air pump is worked rapidly the water will boil actively for a time and then freeze into a cake of ice. The reason for this action is easily explained. The heat required to boil the water could come from but one source, namely, the water itself, and, consequently, after considerable heat had been withdrawn, the water would become so cold that it would freeze.
