qjcends. [Book II. 



nuing, the matter of fire is detached from the air, and 

 combuftion enfues in the fame manner as in the for- 

 mer cafe, when the pure and inflammable airs are fired 

 by the electric fpark. 



Hence there can bs no combuftion without a fup- 

 ply of pure air ; and from this confiderction moft of 

 the phenomena of combuftion may be explained. In 

 a common coal fire, if the coals cake or adhere in fuch 

 a manner that the inflammable part cannot come in 

 contact with the external air, the fire is neceflarily ex- 

 tinguifhed. Flame is ignited vapour; but as that 

 part only which comes in contact with the air can be 

 ignited, that part of the inflammable vapour, which is 

 not confumed, takes the form of fmoke and foot, and 

 adheres to the fide or top of the place or veflel which 

 contains the fire. The flame of a common lamp or 

 candle may be confidered as a tube of fire, in the 

 hollow of which the inflammable vapour is inclofed. 

 It afiiimes a conical form, in confequence of the gra- 

 dual confumption of the vapour, which is leflfened in 

 quantity as it rifes, and 'confequently is contracted in 

 its dimenfions. A confiderable quantity of the va- 

 pour, however, ftill efcapes in the form of fmoke, as 

 muft be evident to any obferver, and as is decidedly 

 evinced by holding a paper, or any other covering, 

 over the flame, in which cafe a quantity of foot will 

 prefently be collected. 



If thefe principles are clearly underftood, it will no 

 longer be a fubject of wonder that all flame naturally 

 afcends. Vapour is confiderably lighter than air, and 

 flame is no other than ignited vapour. Thus in light- 

 ing a common lamp or candle at an ignited bar of 

 iron, or any other burning body, the wick of the 

 candle muft be applied to the lower furface of the 

 ignited body, and not held above it, becaufe then the 



vapour, 



