ON AQUEOUS AKD IGNEOUS METEORS, 707 



Av^lch that ■space contains: and we may easily imagine that the smallest dis- 

 taitce, at which the particles of water, constituting vapour, can exist, with- 

 ou't coming within the reach oftiieir mutual coliesion, is the same, whatever 

 Ocher ^particles .«ia\ be scattered through the intervening space. It appears, 

 howciver, more consistcnit with sofiie cx;perimcnts, to suppose, that the presence 

 of air of the usual density. allows the particles of water to approach a little 

 nearer together without coliering, so that the utmost quantity of moisture, 

 that can be contained iu a cubic foot of air at a given temperature, is not ex- 

 actly the same as would make a cubic foot of pure vapour, but always in a 

 certain proportion to it; and it seems to follow, from the experiments of 

 Saussure, compared with those of Pictet, ihat the weight of the vapour 

 contained in a cubic foot of air is about one half greater than that, of a cubic 

 foot of pure vapour- at the same temperature. 



When the air, iu the neighbourhood of the surface of the water,has become 

 thus saturated with moisture, the evaporation proceeds very slowly, the va- 

 pour being precipitated as soon as it rises; but if the air be continually 

 changed, so that the moistened portion may be removed, and dry air substi- 

 tuted for it, the process will be greatly expedited ; and such a change may bp 

 effected cither by wind, or by the natural circulation, occasioned by any eleva- 

 tion of temperature crmmunieated by the water to the neighbouring air; but 

 when this circulation is jjievented, the evaporation is much diminished, al- 

 though the temperature may be considerably elevated. Iu moderate exposures, 

 theVlepth of the quantity of water, evaporating in 24 hours from any surface, is 

 'ex pressed, ^according to Mr. Dalton's experiments, by the height of the column 

 of mercury ccjuivalent to the force of steam at the given temperature, deduct- 

 ing, however, theeffect of the elasticity of the moisture already existing in the 

 air. 



*Since the quantity of moisture, which the air is capable of receiving, is 

 ■grta.tev ns iis .teiinperature is greater, we may obtain a natural measure of the 

 quantity which it contains, by reducing it to the temperature at which the 

 moisture begins to be r'epositfd. Thus, if we take a glass of cold water, and 

 add to it some common salt, or some muriate of lijrie, we may cool the air 

 near it so TnnciJ, as to cause it to deposit a pait of its moisture on the glass: 

 and by measuiing the temperature uf the water when the precipitation begins. 



