i6' HISTORT of tht SOClEtr. 



liftent with the propagation of heat. Inftances, ftill more re- 

 markable, of the rapid progrefs of fire, are found in the inflam- 

 mation of fuch vapours, when mixed with that proportion of 

 vital air which is necefTary for decompofing the phlogiftic fub- 

 ftance. 



On the whole, Dr Hutton concludes, that in no cafe is the 

 light which appears in burning, an effedt of the heat obtained 

 from the decompofition of vital air, but that it is the extrication 

 of phlogifton, of fixed light, or a certain modification of the fo- 

 lar fubflance, which had exifted in the inflammable bodies, and 

 had been chemically combined with their elements. It appears 

 alfo, that it is light which is immediately produced in burning, 

 and that it is only mediately that heat is excited : This is true 

 both of combufliible bodies which burn in an alTociated ftate, 

 and of thofe inflammable fubftances where the emerging light 

 heats both the inflammable body, and the contiguous atmofphe- 

 ric fluid. 



The Dotftor proceeds to explain, more at large, his notions 

 of the folar fubfl;ance, of which he conceives light, heat, phlo- 

 gift:on, and electricity, to be fo many different modifications. 

 His notions on this fubjedt are very peculiar, as he conceives 

 the folar matter to be without gravitation, without inertia, and, 

 it may be added, without extenfion. The nature of this ab- 

 flraft does not admit of entering further on the argument : It 

 is fufficient to remark, that the theory of heat feems to be 

 arrived at a point where it mufl; almofl; unavoidably ftand ftill, 

 till fome experiments fliall determine how far the gravitation of 

 bodies is afl^edled by the heat, whether fenfible or latent, that is 

 contained in them. The experiments already made, though 

 ingenioufly contrived, and ably executed, are not fufficient to 

 decide a queftion of fuch extreme delicacy ; nor does it feem 

 probable, that, without having recourfe to the pendulum, a fa- 

 tisfadory folution of the difficulty will ever be obtained. 



Phyf. 



