a8 DaHritie if Heat, ivith refpeU ts ienje and elaflic Fluids, 



greater or lefs (legree not only the cohefive but the conftituent afEnities of compound 

 bodies*, In faft, if we confider attentively, we fhall find that we can form no accurate 

 conception of a change produced in the former which (hall not afFe£l proportionally the 

 latter. As the cohefive affinity of a compound body owes its exiftence to the conftituent 

 affinities of its parts, the idea of a diminution in the former includes in itfelf the idea of a 

 correfpondent diminution in the latter. 



Thus, then, we find that every fuggeftlon of analogy would lead us, k priori, to expe£l 

 that a partial or complete difunion of parts fliould always attend the combination of com- 

 pound bodies with caloric ; and, confequently, that their evaporation is to be regarded as a 

 true decompofition. In fubmitting it to the direct teftimony of fadts, however, the phe- 

 nomena of evaporation will be deemed in two refpe£ls irreconcilable with the theory which 

 thefe analogies would fuggeft. The different parts of fuch bodies, it will be faid, do not 

 efcape in that fuccelTion which their different degrees of volatility would thus indicate ; 

 nor does their appearance after the procefs accord with the idea of their decompofition. 

 With refpect to the latter, however, to infer that tliere is no decompofition, becaufe the 

 fubftance collected after evaporation prefents the fame appearances which it had before, is 

 not an admiflible dedu£lion : fince the vapour cannot be fo collefted without having its 

 caloric attached ; and as the theory fuppofes the combination of caloric competent to the 

 difunion, it implies equally that the feparation of caloric will be efFe£tive of the re-union 

 of the component principles. 



But there are alfo pofitive fa£ls to be oppofed to the obje£l:ion. Numerous Inftances of 

 evaporation occur in which we do fee evident marks of decompofition, and of a decompo- 

 fition which we can afcribe to nothing but the combination of caloric. Dr. Prieftley's ex- 

 periments on the generation of air from water, viewed independently of the theory which 

 he connedted with them, afford ftrong illuftrations of it. The ready difunion of the parts 

 of vapour by many fubftances incapable of decompofing it in the ftate of water ; as well as 

 the indications which Count Rumford has noticed of the decompofition of water in 

 operations of qooking, muft be admitted as at leaft prefumptive proofs. And the com- 

 buftive qualities of the vapour of the eolipile, and feveral phenomena of a fimilar kind, 

 prefent evidences of decompofition fufficiently fatlsfaftory f . Nor are the evidences of a 



fimilar 



J 



* I did not kno\y till after I had written the above, that Fourcroy, in fpeaking of the capacity of hydro- 

 gen for caloric, had expreffed himfelf in the following manner: " It even appears that this bafe, though 

 " combined with oxygen in w«ter, ftill poffefles the property of abforbing a great deal of heat. And that 

 " it is this property which renders vapour fpecifically lighter than air." Difcourfe on Modern Cheraiftry. 



•f To thefe may be added the very important inferences to be drawn fiom meteorological phenomena, 

 refpefting the decompofition of water in what is called fpontaneous evaporation. I rauft acknowledge my- 

 felf indebted for much light on thefe phenomena to a very ingenious paper on the fubjeil, in the Effays by a 

 Society at Exeter, and I am difpofcd to differ from the author only on thofe points in which he feems to 

 differ from himfelf. The tnie tenor of his arguments is certainly to prove that water exifts in a decompofed 



ftatfr 



