HISXORr of the SOCIETr. i.5- 



thofe of combuftion. The inconceivable rapidity with which 

 fire is propagated through an inflammable and tranfparent va- 

 pour, is among the moft remarkable of thofe phenomena, and 

 is certainly inconfiftent with the new theory of burning, and 

 indeed with every other that makes fire to be produced by heat 

 alone. Let an inflammable fluid be heated till it boil, and to 

 the top of the column of vapour emitted from it let the fmalleft 

 Jfpark of flame be applied. The vapour is kindled, and, how- 

 ever high the column, the flame defcends in an inftant to the 

 furface of the inflammable fubftance, and fets fire to it. Now, 

 it is impoflible that mere heat covild defcend in this manner, 

 againft the fl:ream of vapour that is continually rifing from the 

 boiling fluid. This is quite contrary to the laws by which it 

 is ufually propagated ; and it fliould feem, that the fadl can on- 

 ly be accounted for by the celerity with which light moves 

 through tranfparent bodies, and by fuppofmg that the extrica- 

 tion of light is the immediate caufe of burning. 



The above inftance is conformable to the experience of every 

 day : Another exampk, which Dr Hutton gives of the celerity 

 with which fire is propagated through an inflammable and 

 tranfparent vapour, is more fmgular, and may perhaps be 

 thought hypotlaetical, but it is. ^t leafl: a very happy application 

 of his theory. This example is the meteor, which, was feen in 

 1783, over all Great Britain, and as far fouth as Pai-is. There can 

 be no doubt, fays he, that this was a. ftream of inflammable var 

 pour which had iflAxed from the mineral regions of Iceland, at 

 that time laid wafte by fubterraneous eruptions. This train of 

 inflammable vapour, about 60 miles high in the atmofphere, 

 had been kindled at the north end, probably by an ekarical 

 fpark, and the inflammation ran the fpace of feveral hundred 

 miles, (at leaft 10,00),, in a minute of time, or little more. 



Thus the inflammation of a body of pure vapour, incontadl 

 ^"^ ^feSu^fM^^^YlS/Jf* is. made with a rapidity; quite incon- 



fiftent 



