CHAP. 2. 21. OF EVAPORATION. Ill 



repulsion, the production of elastic vapour, or 

 gas, is the consequence ; which, exerting its 

 elastic force by the repulsive power of its 

 particles, rises into the atmosphere ; but when 

 a fluid becomes an elastic body, there is a loss 

 of heat of temperature by expansion : and the 

 vapour, therefore, becomes cooler than the 

 water from which it evaporated, and also cools 

 as it expands on its progress, causing the upper 

 air to be cooler into which it ascends ; for it 

 has changed its heat of temperature for heat of 

 capacity;* so that the actual temperature of 

 the air is diminished upwards : while the said 

 gas, possessing heat of capacity, is thus enabled 

 to remain an expanded elastic fluid; and it 

 is only by an actual loss of heat, from the 

 nocturnal interception of the sun's rays, that 

 the whole mass of atmosphere, being cooler, is 

 then again condensed into aqueous particles, 

 and falls in dew; by which process the heat 

 of capacity is again changed for the heat of 

 temperature ; and the reformation of water in 

 the form of mist or cloud, actually increases 

 the thermornetric warmth, in falling ; and thus 



* I adopt the mode of expression of Sir H. Davy. See 

 him in his Elem. of Chem. Phil. vol. i. part i. v. 1, 2, 3, &c. 



