326 COMBUSTION. 



The great discordance which is apparent between the results of these ex- 

 periments shows how much still remains to be done in this department of the 

 physics of heat. It is probable, however, that the results of the experiments 

 of Lavoisier and Laplace are more entitled to confidence than those of the 

 other experimenters. Dr. Thomson thinks that it is probable that one pound 

 of hydrogen gas gives out in combustion as much heat as would melt 400 Ibs. 

 of ice, or 56,000 of heat. 



The copious development of heat, in the process of combustion, and the con- 

 sequent luminous effect, were accounted for by Lavoisier by the fact that a con- 

 densation of matter took place. Thus, when a gaseous substance, by the pro- 

 cess of combination with oxygen, passes into the liquid or the solid state, all the 

 latent heat which maintained it in the form of gas suddenly becomes sensible, 

 and an immense increase of temperature necessarily ensues. The same effect 

 takes place when a liquid passes into the solid state. Now it is certain that 

 in numerous cases of combustion these effects take place ; and all such cases 

 admit of being reduced to the same class of phenomena as the solidification of 

 a liquid, or the condensation of a vapor, in both of which cases, as has been 

 already explained, heat is evolved. Some of the phenomena of combustion 

 may, perhaps, be reduced to the case of ordinary condensation without change 

 of form ; but there are instances which do not seem to fall under this class of 

 effects. On the contrary, in certain cases, solids or liquids, in the process of 

 combustion, pass into the state of gases. Thus, when gunpowder is exploded, 

 the oxygen, which is contained abundantly in the saltpetre, combining with 

 the sulphur and carbon, which are the other constituents of this substance, as- 

 sumes the gaseous form. At the same time a highly elastic fluid is produced, 

 as well as a large quantity of heat and light. 



So far, therefore, as the theory of Lavoisier assumes that combustion is the 

 consequence of rapid chemical combination, and that such combination is ac- 

 companied by a copious evolution of heat and light, it is strictly a statement of 

 fact, but when it is attempted to reduce these facts to the general class of phe- 

 nomena, in which heat and light are developed by condensation, the theory fails, 

 because all the phenomena which it professes to explain cannot be reduced to 

 this class. It is also assumed, in the theory of Lavoisier, that oxygen is a com- 

 pound of heat, light, and a certain unknown base ; that a decomposition takes 

 place by which the heat and light are disengaged, and the unknown base is 

 combined with the combustible. Now the existence of this unknown base is a 

 gratuitous assumption, inasmuch as such a base has never been exhibited in a 

 separate form ; besides which, it is assumed that light and heat are bodies, and 

 not qualities of matter, which is still undecided. 



So remarkable a phenomenon as combustion, and one so susceptible of such 

 various and important practical applications, could not fail, at an early period, 

 to attract the attention of chemists. We accordingly find many theories pro- 

 pounded at various epochs in the history of chemistry for its explanation. One 

 of the earliest of these theories assumes the existence of a first principle, or 

 elementary substance, called fire, which had the property of devouring other 

 bodies. According to this theory, combustion was the process by which the 

 combustible was converted into fire ; whatever part, of the combustible was un- 

 susceptible of this conversion remained behind in the form of ashes. 



Dr. Hook traced the phenomena of combustion to the solvent power over 

 the combustible possessed by a principle found in atmospheric air, similar to 

 one which exists still more copiously in nitre. How near this ingenious hy- 

 pothesis approached to the true principle of combustion may be easily perceived. 

 But the theory which took possession of the scientific world, to the exclusion 

 of all others, for a long period, was the Stahlian theory of Phlogiston. In this 



