32 OP CLOUDS. CHAP. 1. 10, 



modifications; but I have seen it no where 

 proved, that after their formation rain has gone 



wise the vapour from sulphuric ether, which boils at 102, 

 then supporting 30 inches of mercury, at 30 below that 

 temperature it has half the force, and at 40 above it, double 

 the force: and so on in other liquids. Moreover, the force of 

 aqueous vapour of 60 is nearly equal to half an inch of 

 mercury when admitted into a Torricellian vacuum; and water 

 of the same temperature, confined with perfectly dry air, in- 

 creases the elasticity to just the same amount. 



' 3. The quantity of any liquid evaporated in the open air 

 is directly as the force of stream from such liquid as its 

 temperature, all other circumstances being the same.' " 

 The following is a part of the essay on evaporation; 

 " When a liquid is exposed to the air, it becomes gradually 

 dissipated in it; the process by which this effect is produced 

 we call evaporation. 



" Many philosophers concur in the theory of chemical 

 solution: atmospheric air, it is said, has an affinity for water; 

 it is a menstruum in which water is soluble to a certain degree. 

 It is allowed, notwithstanding, by all, that each liquid is 

 convertible into an elastic vapour in vacua, which can subsist 

 independently in any temperature: but as the utmost forces 

 of these vapours are inferior to the pressure of the atmosphere 

 in ordinary temperatures, they are supposed to be capable of 

 existing in it in the same way as they do in a Torricellian 

 vacuum : hence the notion of affinity is induced. According 

 to this theory of evaporation, atmospheric air (and every 

 other species of air for aught that appears) dissolves water, 

 alcohol, ether, acids, and even metals. Water below 212 is 

 chemically combined with the gases; above 21 2 1 it assumes a 



