On the same principle depends sir John Leslie's method of freezing water. The water is placed in a flat porous dish, over a large surface of strong sulphuric acid, and the whole covered with the receiver of an air-pump. When a good vacuum has been produced, there is, of course, as we have already seen, a rapid E.. and the acid eagerly absorbing the vapor as it is formed, the process goes on without further working of the pump, till the residual water has become a solid cake Of ice. A most extraordinary example of this production of cold is afforded by the freezing of water on a white hot plate—by no means a difficult experiment. A platinum capsule is heated nearly to whiteness by a lamp placed underneath; a little water, mixed with sulphurous acid, which is an extremely volatile liquid (indeed it is gaseous at ordinary temperatures and pressures), is poured upon the plate. The acid instantly evaporates, and the cold pro duced freezes the water, which can be dropped from the hot plate on the hand as a lump of ice.
Another remarkable instance of this occurs in the formation of solid carbonic acid. The liquid acid is forced by the pressure of its own vapor in a fine stream into the air from a nozzle in the strong iron vessel in which it is contained. It evaporates so rap. idly in air that a portion of the stream is frozen, and the delicate snow-like mass can be collected by proper apparatus.
Having thus briefly examined some of the circumstances connected with E., we may
proceed to mention some of its important bearings on meteorology. In this respect it is one of the most effective of all the gigantic processes that are continually going on around us. Watery vapor is continually rising invisible in the air ; meeting with a colder stratum of the atmosphere, or the cold ridge of a mountain, it becomes con densed into mists or clouds; the fine par ticles of these unite into larger groups, and fall as rain, hail or snow—to be again evaporated by heat from the moist ground, or from rivers, lakes, and seas. Even when otherwise invisible, its presence may be detected by its deposition as dew (q.v.), and, according to Clausius, in the blue of the sky, and the gorgeous tints of sunrise and sunset. There is little doubt of its being also intimately with the scintillation of the fixed stars. See SCINTILLATION. Atmospheric electricity is largely due to E. directly as well as indirectly, on account of the amounts of vapor contained in different currents of air. It is matter of everyday observation how much the drying of the ground, or E. generally, is promoted by a brisk wind. This finds its explanation in the constant removal of the vapor as it is formed, the diffusion of the vapor taking place into comparatively dry air instead of the moist atmosphere into which it would take place in a calm. See RAIN and Antos PIIERIC ELECTRICITY.