322 COMBUSTION. 



Some one of these four bodies oxygen, chlorine, bromine, and iodine be- 

 ing, almost in every case, one of the two bodies by the combination of which 

 combustion is produced, and the other bodies with which they severally com- 

 bine being far more numerous, the four just mentioned are distinguished rela- 

 tively to the phenomena of combustion by the name supporters of combustion ; 

 while the other body forming the combination with them, whatever it may be, 

 is called a combustible. These terms, however, must be carefully understood 

 as not expressing any distinct or different mode of action which the two com- 

 bining bodies exert in the process of their combination. Supporters of combus- 

 tion and combustibles, as far as has been discovered, have no other difference than 

 this, that the former are very limited in number, and the latter very numerous. 



Exclusive of the four supporters of combustion, every simple substance 

 known in chemistry are combustibles, except azote or nitrogen gas. The mean- 

 ing of this is, that all simple substances are capable of entering into combina- 

 tion with one or other of the four bodies called oxygen, chlorine, bromine, or 

 iodine, in such a manner as to be attended with a sudden evolution of light and 

 heat. 



After the discovery of the true nature of the process of combustion, it was 

 long supposed that the only supporter of combustion was oxygen, and the phe- 

 nomenon of combustion was consequently defined to be the rapid combination 

 of oxygen with some other substance. This is, indeed, the nature of the phe- 

 nomenon in all ordinary cases of combustion ; and it is only in few instances, 

 developed by the researches of modern chemists, that chlorine and the other 

 supporters play a part. 



The tendency which a body heated considerably above the temperature of 

 the surrounding medium has to dismiss its heat, whether by contact or radia- 

 tion, renders it necessary that the combination which produces combustion 

 should be so rapid as to be almost instantaneous ; for, if the heat developed 

 were produced progressively, it would be progressively dissipated, and could 

 never accumulate so as to produce that increased temperature which is neces- 

 sary for the evolution of light. 



In all ordinary cases of combustion, one of the combining bodies is the oxy- 

 gen, which forms a component part of atmospheric air ; and one of the circum- 

 stances which most favor combustion is the fact that the constituent elements 

 of atmospheric air are mixed togther, either mechanically, or, if they be chem- 

 ically combined, their affinity is of the weakest imaginable kind. Thus the 

 oxygen exists in the atmosphere almost in a free state, and ready to combine 

 with any object which presents to it the slightest affinity. The application of 

 heat to any body, by weakening the energy of the cohesive principle, leaves its 

 particles more free to obey other affinities ; and consequently it is found that 

 bodies which cannot combine at om temperature will frequently be capable of 

 combining when the temperature of one or both is raised. A body, therefore, 

 may exist at a certain temperature, when surrounded by the oxygen of the at- 

 mospheric air ; but if the temperature of ihat body be raised, the affinity of its 

 molecules for those of oxygen will at length be enabled to take effect by the 

 diminution of the force by which its particles are held together. In conformi- 

 ty with this principle, we find that when a combustible is raised to a certain 

 temperature, its particles rapidly combine with those of the oxygen contained 

 in the surrounding air. In their combination heat and light are evolved, and 

 fire is produced. When phosphorus is raised to the temperature of 148, it 

 burns with great splendor. The particles of the phosphorus, in this case, com- 

 bine with those of the oxygen in the atmosphere, and so much heat is devel- 

 oped by their combination that the light is evolved. The temperature neces- 

 sary to each different substance, to combine with the oxygen and produce com- 



