41 



0rigin of 

 clouds. 



Eyaporation. 



Vapo"ar. 



The air has no 

 solvent action 

 onift 



Lwfi of the 



natural pro-. 



C«S3. 



THE NATURAL HISTORY OF CLOUDS. 



suddeD change in its electricity : for in tracing the progress 

 of a thunder-storm, through a long range of these clouds in 

 the horizon, we have been sanstied, that the clouds, which 

 had ceased to afford explosive discuarges, had undergone 

 this change in their superior pan, and were pouring down 

 rain ; while others, among which the lightning sliU played, 

 or which were situate beyond it, retained their swelling 

 and rounded forms some time longer. 



Of the Origin, Suspension^ and Destruction o/Clouds» 



These aggregates consist of water, raised by evaporation, 

 and become visible by condensation in the atmosphere. 

 Respecting evaporation, and the state in which vapour sub- 

 sists, there has been much diversity of opinion: and, of the 

 several theories proposed, there is not one comprehensive 

 enough to merit exclusive adoption. A number of general 

 principles, however, have been established ; which we shall 

 employ, with the aid of those of electricity (hitherto not 

 enough considered in its silent and gradual effects), to ex- 

 plain, though in an imperfect manner, the principal pheno- 

 mena of clouds. 



Evaporation consists in the union of water with caloric, 

 and the escape of the compound as an invisible fluid, which 

 we shall exclusively denominate vapour. 



The solvent action of the air, to which this effect has been 

 attributed by chemical philosophers in general, has been 

 proved by comparative experiments on the force of vapour 

 in air, and with air excluded, to have no perceptible share in 

 it. The laws which govern the natural process, for these 

 alone here interest us, may be thus briefly stated. . The 

 force by which water is converted into vapour is directly as 

 its temperature, other things being equal : but this force 

 has to overcome an opposing one, of the same nature, in- 

 herent in the vapour which already exists in the atmosphere. 

 For auch vapour, by its elastic property, tends to exclude 

 from the space it occupies every additional portion ; and 

 <;onsequently to prevent the escape from the water of new 

 v^fpour. Hence the temperatures being equal, the quantity 

 ufvapoHr produced will be less, the greater the quantity 

 already diffused ib the air. 



A.r,^ But 



