ASCENT OF VAPOUR. 183 



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 chymical 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 gene- 

 ral 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. 



That this is actually the case is proved by the 

 fact that in dense air, when that air is warmed, and 

 consequently communicates heat, not only to the 

 surface from which the water is evaporated but to 

 the little drops as they ascend through it, the water 

 rises in visible vapour : and as that vapour mounts 

 into air, containing less and less water as it ascends, 

 and receiving more and more heat in its progress, 

 the little drops are subjected to continual division, 

 so that long before they have risen so high as the 

 top of an ordinary hill, they have become far too 

 minute for observation, and are so dispersed that a 

 gallon of the watery vapour may not weigh one- 

 twentieth part of a gallon of the air through which 

 it is ascending. In that state it is not only impos- 

 sible that it can fall down to the ground, but if must 

 continue to ascend, and to ascend rapidly, in propor- 

 tion as, bulk for bulk, it is lighter than the air. 



And the farther that it ascends, too, it will ascend 

 the more freely and rapidly, and spread to the greater 

 extent ; because the rarer that the air is, the farther 

 must its particles be asunder ; and the higher that 



