192 RAMBLES OF A NATURALIST. 



Water, that element which is almost as necessary as 

 air to the existence of organised beings, enters into 

 the composition of our atmosphere in two different 

 ways. It sometimes becomes entirely invisible, in 

 consequence of actual dissolution, in which case we 

 must have recourse to the instruments known as 

 hygrometers, to reveal its existence ; under other 

 circumstances, its molecules, re-united into small 

 hollow spheres, float in the air in the form of infi- 

 nitely minute vesicles which, by their union, produce 

 those visible vapours which we term mist or clouds. 

 The mere process of cooling is sufficient to cause 

 the invisible vapour to pass suddenly into the latter 

 condition ; for, as in the case of all other gases, the 

 air when cold cannot dissolve as much water as 

 when it is warm. Now when the rays of the sun 

 strike upon the exposed sides of a rugged mountain, 

 the strata of air, which are in immediate contact 

 with this surface, become rapidly heated, and at the 

 same time become blended with a certain quantity 

 of invisible vapour which has been drawn from the 

 soil by the action of heat. Having become at once 

 lighter and more humid, they rise along this inclined 

 plane, forming ascending currents. They cool on 

 reaching a certain altitude, when the invisible vapour, 

 assuming the form of vesicles, becomes suddenly 

 apparent to the eyes of the observer; but new 

 quantities of heated air, being continually added, 

 mix with the cold strata of these elevated regions, 

 and warm them ; and thus vapour, which alternately 

 appears and disappears under the action of these 

 diverse influences, presents all those movements and 



