EVAPORATION. 163 



EVAPORATION. 



IT was long supposed that the vapor produced from the surface of liquids 

 exposed to the atmosphere, was the consequence of an affinity between the 

 particles of air and the particles of the liquid, by virtue of which a combination 

 was formed, and consequently a constant absorption took place by the air, of 

 liquids exposed to it. The properties of vapor, however, which have been 

 discovered by the labors of modern philosophers, and above all, by those of 

 DALTON, have proved the fallacy of this supposition, and have shown that all 

 the phenomena of evaporation may be accounted for without supposing any 

 affinity whatever, or other attraction, to exist between the particles of atmo- 

 spheric air, and those of liquids. 



The explanation of evaporation on the principle of chemical combination of 

 the vapors with air, was first suggested by Halley, and supported by many suc- 

 ceeding philosophers. According to this theory, air was considered as having 

 the same effect on water, as water would have on salt, or any other substance 

 which it might hold in solution. The theory was rendered plausible by the 

 facility which it offered in explaining some of the most obvious phenomena of 

 evaporation, such as the circumstance of its being promoted by winds, and by 

 increase of temperature. Currents of air removing the solvent as fast as it 

 became saturated, brought a fresh portion of it to receive vapor, and so the pro- 

 cess was continued and stimulated. Heat, also, was supposed to increase the 

 solvent power of the air on water, in a manner analogous to that by which it 

 was known to increase the solvent power of water on other substances. 



Vapor, however, at low temperatures, was considered to possess no elas- 

 ticity, and the discovery of the falsehood of this supposition was the first step 

 toward removing the hypothesis of Halley ; but this theory received its death- 

 blow from the fact that vapor is not only formed in a space where no air is 

 present, but that in that space it possesses the same elasticity, and occupies the 

 same volume, as if the same space were filled with the supposed solvent; nay 

 more, that it is not only produced in such a space, but that it is produced in- 



