EVAPORATION. 185 



indeed not only small, but absolutely negative, 

 and entirely obedient to the action of heat ; and 

 not only that, but if air is let into a larger space 

 upon which there is no pressure, it will expand ; 

 and cool, that is, become sensibly cold, or abstract 

 heat from other substances as it expands. And 

 when the quantity of it in a close vessel is di- 

 minished by pumping a portion of it out, and 

 water is placed in the vessel, and some substance 

 is also placed in it which has more attraction for 

 water than the air has, and which in consequence 

 drinks up the vapour of the water as soon as it is 

 formed, the remaining air in the vessel will become 

 so cold that the water will be frozen into a cake 

 of ice, even though the apparatus be in a warm 

 room. 



That simple experiment throws some light upon 

 the very general and important process of evapo- 

 ration. It shows us that when water passes into 

 a state of vapour, or becomes endowed with that 

 dispersive motion of its particles which sends it 

 invisibly through the air, it is really changed to a 

 state very much resembling that of the air ; and 

 thus it may ascend among the particles of the 

 air, in consequence of the dispersive motion which 

 it itself acquires by being heated. So that, though 

 the vapour is invisible, we are not to suppose that 

 it necessarily enters into chemical combination 

 with the air, in such a manner as that the two 

 form one compound substance ; but that it is only 

 dispersed through the air mechanically, and rises 

 by the general law of gravitation, just because the 

 quantity of it which is contained in any given bulk, 

 in a gallon for instance, is less in weight than the 

 quantity of air contained in the same. 



R3 



