OS THE NATURE OF HEAT. 257 



III. 



On the Nature of Heat. By Marsh Alt Hall^ Esq. In 

 a Letter from the Author, 



{Concluded from p. 222.y 



N appreciating the merit of any hypothesis, we ought cer- Some assump- 

 tainly to consider, what assumptions are inseparable ^''^m jj^^p^^^j^ ® 

 the subject itself; and what suppositions are necessary, to from a subject, 

 constitute the particular hypothesis proposed. '^^^tS^^'^h 



To apply this to our subject; it appears to me, that, particular yievr 

 whatever may be our notion concerning the ultimate nature *'^^'* 

 of caloric, one postulate must necessarily be made; the ex- 

 istence of a channel for this agent between the Sun and the 

 Earth must unavoidably be assumed. 



If we embrace the opinion of the materiality of caloric. Suppositions 

 we suppose, that this matter emanates constantly from the w^f'hth? mate- 

 sun's surface; and penetrates space. On the other hand, riality, and 

 in adopting the opposite opinion, we necessarily suppose the ^"^^^*^"**^^y 

 existence of a fluid, naturally pervading the universe in a 

 state of quiescence; but ready to be impressed by external 

 causes. This is indeed the great difficulty ; and a difficulty, 

 which no one will pretend to obviate. It may diminish the 

 objection, which is thus afforded to the hypothesis, to ob- 

 serve, that on either side of the question the difficulty is 

 nearly the same ; or, if there be any difference, it is in fa- 

 vour of the hypothesis of vibration. For what is the great 

 difference, between the assumption of a material agent, 

 which, being impelled, penetrates space with rapid motion ; 

 and that of a quiescent fluid pervading space, and subject 

 to certain impressions ? 



But, if we consider this circutnstance farther, we shall Most assump- 



observe, that, in the material theory, the Assumption of one *^°"^ !".^^^ 

 /, . J , J -. A , . material hypo- 



tiuid only does not sufSce. According to this opinion, the thesis. 



sun-beam must consist of at least three; or, if we consider 

 the compound nature of light, of no less than of nine dis- 

 tinct fluids. Many persons however will be willing to grant 

 all these to modern theorists, who would refuse to Huygens 

 Vol. XX1X,-*AUGCST, 1811. S and 



