42 THEORY of RAIN. 



according to the known laws of condenfation, water muft be 

 feparated from the menftruum, and become vifible by reflecting 

 light. In like manner, water may be rendered an invifible 

 elaftic fluid, by means of heat alone ; and this fluid, in being 

 cooled, will be condenfed into water, and appear vifible. But 

 it is now to be fhown, that, when breath or fleam becomes vi- 

 fible, in mixing with the atmofphere, this effect is not produced 

 in confequence of the general principles of heat and cold : That, 

 for the explanation of this appearance, there is required the 

 knowledge of a particular law ; and that the effects of heat or 

 cold, in relation to air and vapour, proceed not always in ratios 

 which are equally increafing or diminifhing. 



IN order to determine the actual ratio of the difTolving 

 power of air, in relation to water in different degrees of 

 heat, or the ratio in which the power of heat converts fluid 

 water into elaftic fleam, we muft confider the feveral ratios in 

 which this operation may proceed ; for if, among all the con- 

 ceivable ways of proceeding, there fhall be but one with which 

 natural appearances fhall correfpond, it will then be reafonable 

 to conclude, that this correfponding ratio is the particular law of 

 nature, and that appearances of this kind are thus to be explained. 



THE diflblving power of air, in relation to water, may be 

 fuppofed to diminifh as heat is increafed; but this would be in- 

 confiftent with natural appearances in general. Such a fuppo- 

 fition, therefore, would be now fuperfluous. This power might 

 alfo be conceived as not affected with the increafe or diminu- 

 tion of the degree of heat ; and this fuppofition is agreeable to 

 the folution of fea-falt in water : But, as it is certainly not the 

 cafe with air and vapour, neither is this fuppofition to be made. 

 The general rule of diflblving and evaporating bodies, is to in- 

 creafe with heat. This is now to be admitted as the cafe with 

 water evaporating in air, or when, by means of heat alone, it 

 is converted into fleam ; and it is only the ratio or meafure of this 

 operation which here is to be made the fubject of confideration. 



THERE. 



