ON THE NATURE OF HEAT. g£| 



new hypothesis, to reconcile the contradiction: for I CfkB 

 regard the doctrine of capacity in no other light, than'ih 

 that of a second hypothesis, adopted to obviate the imbeci- 

 lity of a former one. 



The objections afforded by this part of the subject, to the 

 general theory, are too palpable to need to be insisted on. 

 If it be argued, that our notion of capacity supposes a 

 power of cotinteracting the usual properties of caloric ; as 

 the properties of acid and alkali mutually neutralize each 

 other; there are facts not less in contradiction to this sup- 

 position. Thus the caloric communicated to boiling 

 water is expanded in satisfying its increased capacity ; ne- 

 vertheless the expansion occasioned is prodigiously great. 



It has indeed been asserted, that there are direct proofs of 

 the existence of material caloric ; it h therefore proper, that 

 we should consider these. 



1st, *♦ The communication of caloric through a vacuum, Comniunica- 

 has been regarded as such a proof." In opposition to this ^'°" ^f heat 

 argument it is sufficient to slate, that no absolute vacuum, cuuh"aot 

 as far as we know, has ever been effected. Cavallo could proved. 

 never render the sound of a bell even perfectly inaudible, 

 although he employed an air pump of the best construction* 

 And in the Torricellian vacuum it is well known, that luj 

 atmosphere of mercurial vapour is formed. Pictet has 

 even observed the condensation of this vapour. By this 

 vapour therefore may the heat be communicated, although 

 not material ; its tenuity affords no objection ; the con- 

 ducting power of bodies does not observe the ratio of their 

 density. ,, 



It is obsei-Ved, that the conducting power of the Torrl- Radiation of 

 cellian vafcuum is to that of the atmospheric air as 100 to l^eat faciliuted 

 r. H.T 1 • /» 7 • . • • by the air. 



o05. ^ow this presents a tact, which it is not easy to recon- 

 cile to the material theory. According to this theory, the 

 radiating power of any body must depend on its own nature 

 and power ; it canriot be assisted, it may be opposed, by 

 Burrounding bodies ; but the fact just stated, and the expe- 

 riments of Mr. Leslie, prove, that radiation is in reality fa-^, 

 cilitated by the surrounding air. 



2dly, ** The radistion of caloric appears to be another une^ Radiatioa of 



quivocal heat 



