1908] on Explosive Combustion. 75 



that for the same gaseous mixture under given physical conditions 

 it always has a constant value. In this connection I would mention 

 Professor H. B. Dixon's exhaustive researches on the ''rates of explo- 

 sion " of gaseous mixtures, which have extended in so many ways our 

 knowledge of explosive combustion. 



Experiment I. — Perhaps the best illustration of the outward dif- 

 ference between ordinary '' inflammation " and " detonation " is afforded 

 by the case of a mixture of carbonic oxide and oxygen in their com- 

 bining ratios. When ignited in an open tube 4 or 5 inches long, the 

 mixture burns quietly with the familiar blue flame. Far otherwise is 

 it, however, when a long column of the mixture is fired in a leaden 

 coil, where the brief initial period of inflammation is succeeded by the 

 explosion ivave, which dashes onwards through the gases at a rate of 

 170U metres (about a mile) per second with shattering effect. 



Another notable feature of " detonation " is the extremely short 

 duration of the flame. In the course of some experiments carried 

 out under Professor Dixon's direction, it was found that the dura- 

 tion of luminosity in each successive layer of gas in the detonation of 

 electrolytic gas does not exceed 3000th part of a second. But short 

 as this time is, it is something like a million times longer than the 

 interval between successive molecular collisions in a gaseous mixture. 



The question of how a hydrocarbon burns, that is to say, precisely 

 how it is attacked by the oxygen, has been the subject of much discus- 

 sion during the past fifteen years. A hydrocarbon is a compound of 

 the two combustible elements, carbon and hydrogen, and in a sufficient 

 supply of oxygen, both of these are ultimately burnt to carbon 

 dioxide and steam, respectively. Thus, for example, in the case of 

 ethane : — 



2C2H6 + 70., = 4CO2 + 6H2O. 



2 vols. 7 vols. 4 vols. 6 vols. 



On kinetic grounds, however, it seems inconceivable that the pas- 

 sage from the initial system of ethane and oxygen to the final system 

 of carbon dioxide and steam can be immediate and direct. It is, 

 therefore, universally recognised that the process involves a number 

 of successive stages. But opinion has been sharply divided as to the 

 nature and sequence of these stages, and I will now endeavour to put 

 before you the main points in dispute. They may be conveniently 

 summarised under three heads. 



1. During the greater part of last century the behef prevailed 

 that the hydrogen is much the more combustible of the two elements 

 of a hydrocarbon, and that consequently when combustion occurs in 

 a limited supply of oxygen, the hydrogen is preferentially burnt, as 

 follows : — 



C2H4 + 0, = 2C + 2H2O 



