186 ASCENT OF VAPOUR. 



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 va- 

 pour 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 gal- 

 lon of the air through which it is ascending. In 

 that state it is not only impossible that it can fall 

 down to the ground, but it must continue to 

 ascend, and to ascend rapidly, in proportion 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 asun- 

 der ; and the higher that the water has ascended 

 the more minutely must it be divided, and the 

 farther must all the parts be from each other. 

 The action of the heat destroys the cohesion of 

 the water with the pool, or the leaf, or other 

 surface from which it rises, the very moment that 

 it begins to ascend ; and the cohesion of the as- 

 cending parts becomes less and less, even much 

 faster than the diminishing size, because of the 

 distance that they are apart, and that is the rea- 

 son why vapour, which is so dense as not only to 

 form a light floating rack, but castled clouds, with 



