ON GASEOUS COMBUSTION. 495 



the hydrocarbon, and less than 5 per cent, of it goes to the hydrogen. 

 Moreover, the proportion of the oxygen which goes to the hydrogen has 

 been found to increase according to the second power of a;, a circum- 

 stance which suggests that the combustion of hydrogen involves the 

 trimolecular reaction 2H 2 +0 2 = 2H 2 0, and not the initial formation of 

 hydrogen peroxide, as some have supposed. A similar series of experi- 

 ments with mixtures CH 4 + 2 +aC0 have proved that the affinity of 

 carbon monoxide for oxygen is even less than that of hydrogen, and 

 that the proportion of oxygen going to it increases as approximately 

 the first power of x. 



These observations have an important bearing on the chemistry of 

 flames ; hitherto hydrogen has been considered as one of the most com- 

 bustible of gases, but in reality it is very much less so than hydro- 

 carbons. Indeed, so overwhelmingly great is the affinity of a hydrocarbon 

 for oxygen, as compared with the affinities of either hydrogen or carbon 

 monoxide or with its own tendency to decompose, that the initial stage 

 of its combustion probably takes precedence of all other chemical phe- 

 nomena in flames. This is certainly true of the propagation of flame 

 through a homogeneous explosive mixture of hydrocarbon and oxygen. 

 In the special case of a stream of hydrocarbon burning in air, partial 

 decomposition may possibly occur in the innermost regions of the flame, 

 where the supply of oxygen is very limited, before actual combustion 

 begins. But in general, whenever the hydrocarbon and oxygen meet at 

 high temperatures, their mutual affinities will prove superior to any dis- 

 ruptive force which might otherwise break down the hydrocarbon. It is 

 probably not so much the original hydrocarbon as its hydroxylated mole- 

 cule which decomposes in ordinary flames. Be this, however, as it may, 

 the experimental evidence does not warrant the view, so often encoun- 

 tered in scientific literature, that hydrocarbons are resolved into their 

 elements prior to being burnt. 1 



Section VI. — The Influence of Hot Surfaces upon Combustion. 



It is to the genius of Sir Humphry Davy that we owe the discovery 

 of surface combustion. In his experiments on the ignition-points of 

 various gases he had found, what is now a matter of common knowledge, 

 that the constituents of a combustible mixture will combine with fair 

 velocity at temperatures below the ignition -point. This led him to ask 

 the question whether, seeing that the temperatures of flames far exceed 

 those - at which solids become incandescent, a metallic wire can be main- 

 tained at incandescence by the slow combination of two gases without 

 actual flame. He therefore tried the effect of introducing a warm 

 platinum wire into a jar containing a mixture of coal gas and air rendered 

 non-explosive by an excess of the combustible constituents. The wire 



1 No attempt has been made in this report to discuss in any detail the experi- 

 mental evidence on which the new theory of hydrocarbon combustion is based ; the 

 reader is referred to the series of papers published by the writer and his pupils (R. V. 

 Wheeler, W. E. Stockings, G. W. Andrew, Julian Drugman, and H. L. Smith) in the 

 Trans. Chem. Soe. (1902, 81, 535; 1903, 83, 1074; 1904, 85, 693. 1637; 1905, 87. 910. 

 1232 ; 1906, 89, 652, 660, 939, 1614). 



k k 2 



