Combustible Substa7ic* . 199 



As the contact and succession of fresh air is 

 necessary to the combustion of bodies, we find 

 that it takes place always on the surface. Thus 

 a piece of charcoal is only consumed on the sur- 

 face ; for if we break it, we find it still the same 

 in the middle. Thus, spirit of wine burns at its 

 external surface, for a cotton wick remains with- 

 out change. 



Of combustibles, some burn without, and some 

 with flame. 



The first are, charcoal of various kinds, me- 

 tallic bodies, if they may be classed as such, &c. 

 The second are very numerous, as sulphur, oils, 

 spirit of wine, &c. The proper distinction be- 

 tween these classes is, that the one is fixed, and 

 the other volatile. In the fixed body, the con- 

 sumption can only happen on the surface, while 

 those that are volatile are converted into vapour 

 in a heat below that necessary for their inflam- 

 mation ; and consequently the vapour only can 

 be heated to a proper degree and set on fire. 

 Flame is only a succession of burning vapour, 

 therefore it is always in some degree conical ; 

 for it is consumed as it ascends, therefore di- 

 minishes from a broad basis, and terminates in a 

 point. This form is most observable in the 

 flame of spirit of wine. The vapour, as it rises, 

 is not wholly consumed; the reason is, the air 

 has not free access to the centre of the column 

 of vapour. The combustion, therefore, takes 

 place where the air has freest access, viz. on the 



