I 279 ] 



Section I. 



Of the Influence of Heat. 



The connexion between heat and evaporation is too obvious to 

 have efcaped the notice of even the mofl inattentive to the ope- 

 rations of natural caufes. Every one knows that, in general, the 

 greater the heft applied to water in fitiiilar circumftances, the 

 more quickly it will be converted into vapour. As aqueous 

 •vapours are by far the moft abundant in the atmofphere, to their 

 confideratiOn I chiefly attend in this EfTay. I have faid in general.^ 

 becaufe Leidenfroft's experiments, and fome others which will be 

 mentioned in the fequel, fl:iew that this law muft be circumfcribed 

 within certain limits. 



That all liquids are expanded by an increafe of heat is alfo 

 well known. This rarefadion is oppofed by two powers, namely, 

 the attradion of their integrant molecules to each other, and that 

 of external prefTure. But there is fcarce any degree of heat fo 

 low, as to be totally difabled to overcome the refiftance oppofed by 

 the power of integrant attradion ; as appears by various experi- 

 ments, in which, external compreflion being dimlnifhed or with- 

 drawn, the rarefadion and evaporation could arife from heat fingly. 

 Thus SaufTure found that water is more readily converted into 



M m 2 vapour 



