9<5 Caloric an elementary Subftance* [Book II* 



from a concave mirror* of two feet in diameter ; in 

 confeqnence of which the vane or copper plate, moved 

 on repeated trials with a gradual motion, of about one 

 inch in a fecond of time. This experiment I think 

 a iufficient demonftration, if any demonftration was 

 wanted, that light at lead is a fubftance. Of the iden- 

 tity of light, heat, and fire, I fhall have occafion after- 

 wards to treat. 



4thly. The electric fire affects bodies with a true 

 corporeal percuflion * ; and that this effect is not ow- 

 ing to the vibration of the air, or any medium but that 

 of fire itfel is proved by many experiments in vacuo, 

 &c. Now, if one fpecies of fire is allowed to be ma- 

 terial, there feems to be no reafon why we mould 

 deny the fame attribute to the reft. 



5thly. It is not eafy to conceive how a body can be 

 expanded by motion alone ; and it is much more na^ 

 tural to fuppofe, that bodies are expanded by the in- 

 terpofition of an extremely active and elaftic fubftance 

 .bet>veen their component particles. 



6thly. It is well known that there can be no igni- 

 tion or combuftion, that is, there can be no very high 

 degree of heat, without a fupply of air j a candle, for 

 inftance, will ceafe to burn in vacuo, or in air, the 

 pure part of which is dcftroyed by burning or refpi-- 

 ration. This is a fact which cannot be accounted for 

 on the principle that ail heat is ho other than inteftinc 

 motion ; but is eafily explained it we fuppofe fire a 

 tliftinct elementary fubftance, which is contained in 

 pure air, and is yielded by the air co the force of a fu- 

 perior attracti 



ythly. Thai heat is generally accompanied by mo- 

 tion, is no proof that heat and motion are the fame j 



- Jones's Phrfibl. Difq. p. 8j. 



oa 



