EVAPORATION, is the process by which the aque ous particles of bodies are converted into vapour. This process is of two kinds, Artificial, and Natural or Spontaneous. We have already described the general appearances connected with the former, (See CHEMIS arid we shall therefore confine our attention at present to the consideration of the latter.
Spontaneous evaporation is promoted by a variety of causes, of which the principal appear to be, elevation of temperature, and the successive application of fresh portions of air. Neither of these circumstances, how ever, can be deemed essentially necessary to the pro cess; since ice gradually wastes away at a very redu ced temperature, and even more rapidly under an ex hausted receiver, than when it is exposed to a current of atmospheric air.
Various theories have been proposed, with the view of explaining the conversion of water into vapour, at natural temperatures, and its subsequent elevation in the atmosphere ; but notwithstanding the frequency with which the process is presented to our observation, and the important purposes which it is destined to serve in the economy of nature, there are few subjects of philosophical enquiry involved in greater obscurity. According to Des Cartes, the action of the sun upon the water converts small particles of that fluid into hol low spheres, which, being filled with a subtile matter, are rendered specifically lighter than air, and thus ascend in the atmosphere. Desaguliers, assuming the hypothesis that heat acts more powerfully on water than on common air, asserts that a temperature, which by its cold condenses air, may be sufficient to cause an evaporation from water, or even from ice. He farther assumes, that the particles of water, after they are converted into vapour, acquire a repellency to one ano ther, and deriving elasticity from the contiguous air, recede farther and farther, till the specific gravity of the fluid which they form becomes lighter than air, after which they ascend. The particles of the vapour are supposed to retain their repellant force, till by the dimi nution of the density of the surrounding air their re lative weight is increased, and they again descend in •the form of rain, hail, Ste.
A theory somewhat similar to this has been support ed, with much ingenuity, by Dalton and De Luc. These philosophers maintain, that since water passes readily into vapour in vacuo, where the agency of the air is completely excluded, its spontaneous evaporation in the atmosphere may be referred entirely to the operation of caloric. Accordingly, Mr Dalton, agreeably to his opinion respecting the constitution of the atmosphere, asserts, that the aqueous vapour, thus formed by heat alone, exists in air, not in a state of combination with it, but merely of mixture or. diffusion ;—that it exerts no action whatever with the surrounding gasses, but sup ports itself entirely by its own elasticity ;—and that the quantity of it depends entirely upon the temperature of the air, and the pressure exerted by the vapour already formed. He even advances a step beyond this, and af
firms that the quantity of vapour which could exist in the atmosphere would be the same, though the pres sure of the atmosphere did not exist, as the vapour itself would soon accumulate, and form an atmosphere which would produce on the surface of liquids all the mechanical effects of the air itself.
The principal argument which Mr Dalton has brought forward in support of his .theory, is drawn from the well-known fact, that water passes readily into vapour under an exhausted receiver, and where, since there is no air present, the evaporation must be ascribed entire ly to the influence of caloric. Dr Murray has shewn, however, that it by no means follows, because water passes into vapour in vacua, at a natural temperature, it will pass into vapour to the same extent, at the same temperature, under the ordinary pressure of the atmo sphere. The two cases are indeed totally different. When water is placed in vacua, evaporation cannot, it must be admitted, be owing to the presence of air ; but it ought to be recollected, that when the air is removed, its mechanical pressure is removed along with it : and it would be altogether illogical to conclude, that because water passes into vapour in the absence of that pres sure, it would do so in an equal degree, when exposed to it. "The proper manner of making the experiment," as Dr Murray very properly remarks, " is to exclude the chemical agency of the air, while the pressure of it is preserved; in other words, to place water in vacua under the pressure of a column of mercury 29 inches in height, which is equal to the pressure of the atmo sphere. If in this case any vapour were formed, the conclusion would be just, that spontaneous evaporation is independent of any chemical agency of atmospheric air. But the fact will be found very different ; for although_ water introduced into the tube of the barometer at 60° passes into vapour, and depresses the mercury half an inch, that vapour will be completely condensed, and the water retained in the fluid form by a pressure a little greater, instead of being able to exist under the pres sure, which is equal to that of a column of mercury 29 inches high. Mr Dalton has endeavoured to repel this objection, by maintaining, what appears to be quite in consistent with fact, that pressure does not prevent the evaporation of fluids, or at least, that the pressure of the atmosphere is exerted in such a way as to allow, by its partial action, the escape of some of the particles of fluids : for, according to his opinion, " it is not till the depth of ten or twelve strata of particles of any liquid, that the pressure upon each perpendicular column be comes uniform ; and that several of the particles in the uppermost stratum are in reality subject to but little pressure." It is unnecessary to observe that this as sertion is entirely hypothetical, and even at variance with the known laws of hydrostatics.