There is no subject that has occasioned a greater variety of opinions than the theory of evaporation ; but, consistently with our plan, we shall recite only a few established facts.
Evaporation is one of the great chemical processes by means of which Nature supplies the whole vegetable kingdom with the dew and rain necessary for its support. Hence, it takes place at all times, not only from the surface of the ocean, but also from that of the earth. Nor is it confined to these : it is even carried on from the leaves of trees, grass, etc. with which the earth is covered. Great part of the water which is thus raised, descends again during the night, in the form of dew, being absorbed by those vegetables which yielded it before.
One of the most beneficial effets of evaporation is, to cool the earth, and prevent it from being too much heated by the sun. This property of producing cold, by evaporation, has but.lately been observed by chemists, who have accordingly availed themselves of it in its fullest extent; though their mode of procuring cold, by means of those expensive fluids,ether and spirit of wine, can only be em-ployed by way of experiment. The most simple method, however, of producing cold by the evaporation of water, may be applied to various useful purposes, especially in warm countries: thus sailors are accustomed to cool their casks of liquors, by sprinkling them with sea-water. - See also Ice.
Dr. Darwin justly observes, that the evaporation of moisture from the surface of the earth, pro-duces so much cold as to injure those terrestrial plants which are too long covered with it. Hence such parts of wall-trees as are sheltered from the descending dews, by a coping stone on the wall, are not so liable to be injured by frosty nights; because they are not made colder by the evaporation of the dew, and also have less water to be congealed in their vessels, and to burst them by its consequent expansion.