•,f6 JDoUnne if Heat, with refpeSl it denfe end elajtic Flmdt. 



produced in the prefence of air by a lefs quantity of caloric than is required in its abfence. 

 The contradi£lion of this, by experiment, is explained by telling us the air, being after a 

 Ihort time faturated with water, aOiS merely by its mechanical prefTure, and that to over- 

 come this prefTure the vapour requires the combination of a greater proportion of caloric. 

 Yet, after all> we find that vapour in the formation of which this exceflive quantity of 

 caloric has been employed, manifcfts confiderably lefs caloric in its condenfation than 

 vapour produced in vacuo with a heat of 70°. 



In what, then, have we reafon to believe the a6tIon of caloric in evaporation to confift ? 

 From what has been already faid, and particularly from the experiment juft noticed, we 

 have, I think, reafon to conclude that no known phenomena authorize us to refer It to any 

 dire£t fpecific combination. Were this fuppofed, it would furely be expelled that we 

 fhould affign fome determinate proportion to fuch combination. If this cannot be done, 

 if we can in no refpe£l fo proportion the caloric applied that it (hall uniformly produce 

 the fame modification, however we may think ourfelves able to account for this difference 

 of effedt, our explanation muft inevitably imply that there is fome condition more efTential 

 to evaporation than the union of caloric. Thus, as in the explanation of the impediment 

 which evaporation fuftains from the prefTure of the air, it cannot be pretended that this 

 afts by preventing in the firft inftance the dire£l: combination of caloric, but by preventing 

 this combination from producing the efTefts which would otherwife accompany it, we are 

 fully authorized to conclude that the dire£l combination of caloric is not, as the ordinary 

 theory of this phenomenon, and as all analogy drawn from the phenomena of liquefa£lIon 

 would imply, the determinate fpecific caufe of evaporation. 



It becomes interefting, therefore, to inveftigate that more efTential condition of this 

 phenomenon, which the common character of all the circumftances which modify it in- 

 dicates to be expanfion : and, fince we find that it is by the degree in which this condition 

 is induced, and not by the quantity of caloric aftually prefent that the agency of the latter 

 in evaporation is to be meafured, and fince we find that expanfion has a-conftant tendency, 

 even Independently of caloric, to caufe evaporation, we muft expeft to derive, from tracing 

 the modifications which this condition implies, our moft comprehenfive ideas of the prin- 

 ciples of thefe phenomena. 



Viewing the expanfion of a body in Its moft fimplc light, merely as indicating a dimi- 

 nution in the attraftlve tendencies of Its particles, there are two ways in which our 

 knowledge of thefe tendencies fuggefts to us that It may be effe£ted : one, by a change in 

 the ftru£ture or configuration of the particles themfelvcs ; the other, by a change in the 



* The influence of the air has indeed been an equal fubjeft of perplexity with thofe who have attempted 

 to aflbciate it with the agency of caloric in evaporation, and with thofe who have excluded it from this 

 aflbciation. If we would believe the moft celebrated advocate of the latter opinion, it is alike to the 

 mechanical influence of the air (for he denies it to have any influence but mechanical) that we are to attribute 

 the liquefaftion of vapour at one degree of prefTure, and its power of relifUng liquefaction when fubjefted to 

 a greater. See Dc Luc's Theheocalagie, Vol. I. p. 20, jfc. 



3 mutual 



