COMBUSTION. 



327 



theory, the phenomenon of combustion was explained by assuming the exist- 

 ence of a body called phlogiston, which was supposed to be a constituent ele- 

 ment of all combustibles. The process of combustion consisted in the sudden 

 separation of phlogiston from the combustible ; and this separation was accom- 

 panied by the heat and light which characterized the phenomenon. Some suc- 

 ceeding philosophers regarded this phlogiston as light maintained in bodies, as 

 it were, in the latent state, and with its ordinary concomitant heat. Dr. Priestley 

 and others discovered that the atmospheric air in which combustion takes place 

 becomes incapable of permitting the same phenomenon to be repeated in it, and 

 likewise that such air was rendered incapable of supporting animal life. He 

 inferred that atmospheric air had an affinity for phlogiston, and that its presence 

 was necessary, in order to effect the extrication of phlogiston from the com- 

 bustible, and, consequently, that the presence of atmospheric air was essen- 

 tially necessary to combustion ; but that when the atmospheric air became sat- 

 urated with the phlogiston which it received during the process of combustion, 

 the same air, being incapable of combining with any greater quantity of phlo- 

 giston, was incapable of sustaining the process of combustion. 



Still the phlogistic theory labored under the capital defect, that the exist- 

 ence of phlogiston as a separate principle was never proved ; and, in fact, that 

 the assumption of its existence had no other foundation than its convenience 

 for the solution of the phenomena of combustion. This defect in the theory of 

 Stahl was attempted to be removed by a bold assumption of Kirwan, viz., 

 that phlogiston was no other substance than hydrogen. The necessary conse- 

 quences of the adoption of such an hypothesis were, that hydrogen is a compo- 

 nent part of every combustible body ; that combustion consists in the decompo- 

 sition of the combustible into the hydrogen and its base ; that, after issuing from 

 the combustible, the hydrogen combines with the oxygen of the atmospheric 

 air. Such were the bases of the Kirwanian theory. 



Matters were now ripe for the discovery of Lavoisier. Hook had held that 

 a principle in atmospheric air, identical with the prominent element of salt wa- 

 ter, was a solvent for all combustibles ; that the solution effected by it was ac- 

 companied by heat and light. Kirwan held, that a combination of a certain 

 element of the combustible with the oxygen of the atmospheric air was the 

 cause of combustion. Lavoisier, rejecting what was superfluous in these theo- 

 ries, at once assumed that combustion was caused by the combination of the 

 oxygen of the atmosphere, not with hydrogen, or with the imaginary substance 

 of phlogiston, but with the combustible itself, and that in such combination heat 

 and light were produced. He accounted for the phenomena by two admitted 

 chemical laws : first, that the chemical affinity of bodies for each other is awa- 

 kened by the elevation of temperature of one or both ; and, secondly, that a body, 

 in passing from the gaseous to the liquid or solid state, produces an abundant 

 evolution of heat. The combustible, therefore, when raised to a certain tem- 

 perature, is brought to the state in which its chemical affinity for oxygen is 

 capable of taking effect. The oxygen in combining changes its form, and dis- 

 engages a large quantity of latent heat. 



This theory was quickly embraced by Berthollet, Fourcroy, Morveau, and 

 other leading chemists of the times, and has since been very generally received. 

 There are, however, as has been already stated, some phenomena connected 

 with combustion which it fails to explain. These are the cases where, in the 

 combustion, the change of form is the reverse of that which, according to the 

 theory of Black, would cause a development of heat. When the combining 

 substances previously exist in a solid state, and during combustion pass into the 

 gaseous state, we should expect a large absorption of heat, instead of a consid- 

 erable evolution of this principle. 



