TRANSACTIONS OF SECTION B. 025 



light on the question at issue. It is my intention to extend the work to other 

 typical hydrocarbons in the hope that the gradual accumulation of experimental 

 facts may at some future time provide a sure basis for a general theory of hydro- 

 carbon combustion. Meanwhile, it seemed to me that the meetings oi' the 

 Chemical Section afforded a fitting opportunity of communicating and discussing 

 these new observations, of obtaining suggestions for future work, and possibly 

 also, of arranging some form of co-operation among those workers who are specially 

 interested in this iield of inquiry. 



It seems to me unnecessary to make more than a passing reference to the 

 theories which up to the present have been advanced to explain the mechanism of 

 hydrocarbon combustion. I must, however, say a word with regard to two of 

 them which involve the idea of the ' preferential ' combustion, either of hydro"'eu 

 or of carbon. The older idea, that in a defective oxygen supply the hydrogeiT of 

 a hydrocarbon burns preferentially to the carbon, is unsupported by experimental 

 evidence, and I suppose now hardly finds acceptance among chemists at any rate. 

 On the other hand, the opposite view, that the carbon burns preferentially to the 

 hydrogen, was put forward, originally by Kersten in 1861 I believe, to explain 

 the well-known fact that when such a hydrocarbon as ethylene is exploded with 

 just sufficient oxygen to burn the carbon to carbon monoxide, the cooled products 

 consist of carbon monoxide and free hydrogen — 



C2H^ + 03 = 2CO + 2H,. 



Professor Smithells in 1892 was led to indorse this view as the result of his 

 analyses of the interconal gases of hydrocarbon flames. 



It seems to me that the idea of ' preferential combustion,^ whether of hydrogen 

 or of carbon, is closely allied to the old doctrine of ' elective affinity' and that it is 

 liardly to be reconciled with modern conceptions of the nature and conditions of 

 chemical change in a homogeneous system. Furthermore, it may be pointed out 

 that the evidence usually adduced in support of the contention that carbon bums 

 preferentially to hydrogen is wholly derived from experiments on the oxidation 

 of hydrocarbons at very high temperatures, either in the flame, or in the explosion 

 wave. Under these conditions it is practically impossible, by any means at our 

 command at present, to distinguish the character of the: primary oxidation in the 

 case of a hydrocarbon, for since the velocities of all the reactions concerned 

 are enormously gi-eat, the firal state of equilibrium is almost instantaneously 

 established. 



The experiments on the slow combustion of methane and ethane, which have 

 led me to make this communication, have been carried out at temperatures far 

 below the ignition-points of the gases— that is to say, at temperatures where the 

 oxidation velocities are sufficiently small to allow of their being easilv measured. 

 It is also important to observe that either of the hydrocarbons in question 

 interacts with oxygen at temperatures below those at which the velocities of any 

 of the undermentioned possible secondary changes become appreciable : 



(i) 2H., + 0„ = 2H,0. 



(ii) 2CO + 0.>2CO.,. 



(moist) ' 

 (iii) CO + H.0 ;;t C0„ + H,0. 

 (iv) Reduction of COj, or of H.,0, by carbon. 



Therefore, by suitably choosing our temperature conditions we have been able 

 to exclude the possibility of these reactions occurring, and so to prevent the com- 

 plete masking of the primary reaction by secondary changes. 



The details of these experiments either have been, or shortly will be pub- 

 lished elsewhere,' and we need therefore only here indicate the general character 

 of the results. 



In the first place, we should say that the mixtures of methane (or ethane) and 

 oxygen employed usually contained just sufficient oxygen to burn the carbon of 



' Tram. aiem. See. (1902), 81, 535: 83, 1903. 

 1903. 



