314 OUTLINJES OF NATURAL PHILOSOPHY. 



Rain. 



424. The vapour that rises from water uniting 

 itself to the air, ascends into the higher regions of 

 the atmosphere, and is carried by the winds to great 

 distances. 



It cannot be doubted, that the humidity raised in this 

 manner is chemically dissolved in the air. 



The humidity does not lessen, but increases the trans- 

 parency of the air, and cannot be withdrawn from 

 it but by substances which attract it powerfully. 



425. The power of air to dissolve humidity in- 

 creases in a greater ratio than that in which its 

 temperature increases. 



It appears from SAUSSURE'S experiments, that while 

 the temperature increases in arithmetical progres- 

 sion, the humidity which the air is able to contain 

 increases in geometric progression. A cubic foot 

 of air, of the temperature 66, is able to hold in 

 solution 11 or 12 grains of water : the air itself 

 weighs 570 grains : so that air of the temperature 

 66, dissolves about a 50th part of its own weight 

 of water. Essai sur THygrometrie, xi. chap. x. 

 p. 167., &c. 



426. Hence, 



