On the Nature of Heat. 3 1 9 



we are entitled to prclume that, where a contrary effeft is 

 witneilcd, it proceeds from the expuliion of feme fluid or 

 matter foreign to the fiibftance on which it atls,) is another 

 fadl wholly inconfiftcnt with the theory in review. Thus, 

 \vhen we perceive that heat, applied to anv fubltance, inva- 

 fiably occafions its dilatation, we are juftified in concluding 

 that it is enabled to produce this ctlei^t only by virtue of its. 

 own proper cxtenfion ; and if we allow that it is capable of> 

 extcnfion, we cannot rcfufe our afl'ent to its being a bodily 

 fubltance. if we fuppofe this eB'ect to be produced in any 

 other wav, as by the impreflion of any mechanical motion 

 on the parts of the fubltance, it is difficult to conceive how 

 this could produce a lading cxpanfion of the panicles; it 

 would excite, indeed, an unduiatory or vibratory eftecl, occa- 

 fioninsr a change in the lituation of the particles for the time, 

 but there would ftill be a tendency in them to return again 

 to their former (ituation, and an ofcillatory effe£l only would 

 be produced, perteC:tlv diflinc.1 from extenfion. It is, how- 

 ever, inipoffiblc to reconcile this fact with the fuppofition 

 that heat is a quality, or an adventitious and acceitory pro- 

 perty refuhing from the intefiine motion of the particles of 

 bodies, and not a fubllance of itfelf. " Whenever," fays 

 Mr. Locke, " wc perceive a number of qualities always ex- 

 ifting together, we are warranted in the conclufion that there 

 is fome fubftance which produces thofe qualities." If heat 

 dei)endcd on motion, it is natural to expeft that it would di- 

 fplay a greater fiicilitv in its paHage through eladic bodies than 

 thofe that were more dcnfe ; yet wc find that no rule of this 

 fort obtains, but that it pall'es through the former equally 

 flow with the latter. Again, if heat be generated by motion, 

 it is reafoiiablc to expctl that its laws of propagation would 

 be always analogous to the latter; yet it is hardly neceflary 

 to obferve how few thincs we are more ignorant of than the 

 rule which takes place in the proiireHion and communication 

 ot heal in bodies of unecjual temperatures. 



Thole who have ado])tcd the theory of vibrations have af- 

 fnmcfl a polition which there is no fatisfa6tory evidence to 

 jiiltifv their doiufj ; for it does not appear in any one inllance 

 that we are able to demonftratc the exidence of vibrations in 

 healed bodii s, while certain founds will cauie the mv)d iolid 

 fubdiiuices to Vibrate perceplibly without any heat being ap- 

 parently produced. The ingenious experinieius of Count 

 llumford, it will not be dilp\ited, have at leaJt proved that 

 fluids are verv imperfciH condu<i:tors of heat. That they arc, 

 as allumcd by the Count, perfed-t non-condu6tori, iii an in^ 

 fereucc that cannot Icgitimaicly be di'awn even irom his own 

 5 reai'onings j 



