3© Do^rine efiffeat, with re/pe^ to denfe and elafltc Tlutit. 



It requires little attention to dlfcern that the fancied analogy between the cold which is 

 produced in evaporation and in liquefaftion, has in reality nothing for its fupport but the 

 generality of occurrence in each phenomenon. The cold which takes place in liquefaftlon 

 is a mere confequence of the fubjedl of the liquefa£tion being furrounded by bodies of a 

 higher temperature than its own, whofe caloric therefore is in fome part detached, on the 

 general principle of its tendency to equilibrium *. In evaporation, on the contrary, the 

 circumftances are nearly the reverfe. The fubje6l of this procefs has in general a higher 

 temperature than that of the fubftances around it. The tendency to equilibrium between 

 thefc will not account therefore, as in the cafe of liquefaftion, for the cold which takes 

 place. We muft look forward for the occafion of it to the produdt of the operation. 



We (hall be affifted in our examination of this by premifing, that we have no inflance 

 in the whole phyfical hiftory of nature of a fubftance producing cold in confequence of 

 any difference between its capacity and that of the furrounding bodies, unlefs it have at 

 the fame time a temperature aftually inferior to their's. I have already fliown that the 

 cold produced in the liquefa£lion of ice is to be referred to this : and we may eafily fatisfy 

 ourfelves that in no frigorific phenomena whatever have we reafon to afcribe it to any 

 thing clfe. Thus in mixtures of falts and fnow, it is not the change of capacity which is 

 the proximate caufe of the attra£lion of caloric from furrounding bodies. It is merely the 

 remote caufe of it by the redu£tion which it occafions in the temperature. By this change 

 of temperature only are we able to judge of the nature and extent of the change of capacity, 

 and the accuracy of this idea is clearly demonftrated by the uniformity of its correfpondence 

 with the change which we find produced in the freezing point of the new compound. 



From thefe confiderations we have a right therefore to infer, that the vapour of water, 

 ether, &c. cannot produce cold in the furrounding bodies without having a temperature 

 aftually inferior to their's. But we know that the evaporation of thefe fubftances produces 

 cold when the temperature of the furrounding bodies is greatly inferior to that which is 

 univerfally deemed eflential to the fubfiftence of thefe vapours. We know that, were it 

 pofllble for the vapours of water, or of ether, to fubfift at the temperature here implied, 

 inRead of fubtra£ling caloric from fuch bodies, they would impart their own to them and 

 undergo condenfation. We know alfo that, if it were poffible for the vapour of water 

 (for inftance) to fubfift at a temperature as greatly inferior to that of 2ia°, it would have a 

 much greater affinity for the caloric of the evaporating water than the water itfelf : of 

 courfe that, whilft fuch vapour was prefent, the evaporation could not proceed, which is 

 contrary to fa£l. We are inevitably led to conclude, therefore, that the cold here pro- 

 iluced, is not produced by che vapour (underftanding the word in its ordinary acceptation) of 

 die fubjcil: of the operation. Nor do I conceive how it is to be accounted for, but by afcribing 



* The cold in this cafe is inaccurately faid to be the confequence of change of capacity. This change of 

 capacity merely determines the extent to which the proximate caufe, the inequality of temperature, ihall 

 operate. This diftin£tion, though as obvious, is not fufEciently attended to. 



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