March i8, 1909J 



NATURE 



83 



With the extension of the research to the conditions 

 existing in hydrocarbon flames and explosions, it became 

 increasingly evident that the mechanism of, combustion is 

 essentially the same above as below the ignition point. 

 I do not mean, of course, that the phenomena observed at 

 low temperatures, in slow combustion, are exactly repro- 

 duced in flames, but rather that the result of the initial 

 molecular encounter between the hydrocarbon and oxygen 

 is probably much the same in the two cases, namely, the 

 formation of an " oxygenated " molecule. At the higher 

 temperatures of flames, secondary thermal decompositions 

 undoubtedly come into operation at an earlier stage, and 

 play a more important ni/e than in slow combustion, but 

 thev do not precede the onslaught of the oxygen upon the 

 hydrocarbon, but arise in consequence of it. 



Having thus explained the main issues, I propose to 

 perform a series of experiments on the explosive combustion 

 of acetvlene, ethylene, and ethane, some of which are 

 crucial as regards the rival theories under discussion. 



Experiment II. — I have here three cylindrical bulbs of 

 stout borosilicate glass (capacity = about 60 c.c), fitted with 

 firing wires, hermetically sealed, and containing respec- 

 tively equimolecular mixtures of each of the three hydro- 

 carbons with oxygen, that is to sav, mixtures correspond- 

 ing to C,H,-l-0,, C,H,-(-0„, and C,H,-hO,, respectively. 



Now, according to the theory of the preferential com- 

 bustion of carbon, these mixtures should on e.\plosion 

 yield nothing but carbonic oxide and hydrogen, without 

 any separation of carbon or formation of steam, as 

 follows : — 



(a) C.,H.,-l-Oo = 2CO + H.. 

 {/') C.,Hj + 0., = 2CO + 2lf., 

 (.) C.,H5 + 0., = 2CO+,3H., 



. 15 



. 2'0 

 . 25 



* The symbols /) and/.,, uiied in this and subsequent tab'es, denote the 

 initi.tl .-ind tinal pressures uf the ro/(/ original mixture and gaseous product-; 

 (dr>') at constant volume and at the same temperature. 



On firing the mixtures, it is at once evident that some- 

 thing very like this does happen in the cases of (n) and 

 (()). There is absolutely no deposition of carbon, and no 

 appreciable condensation of steam in the cold products. 

 Far otherwise is it, however, in the case of the bulb 

 containing the mixture C,H,-(-0,. A lurid flame fills the 

 vessel, accompanied by a black cloud of carbon particles, 

 and a close inspection of the cold bulb will reveal a con- 

 siderable condensation of water. The pressure ratio 

 f>„//>, is approximatelv 1-5, and an analysis of the gaseous 

 products would prove the presence of about 10 per cent. 

 of methane. The bulb will now be opened, rinsed out 

 with water, and the formation of aldehydic products 

 demonstrated by means of Schiff's reagent. It is clear 

 that these results are wholly inconsistent with the theory 

 of the preferential burning of carbon. 



Did time permit, I could easily demonstrate to you by 

 other similar experiments that the outward difference here 

 revealed between the burning of ethylene and that of 

 ethane extends to all the other gaseous olefines and 

 paraffins; that is to say, whereas mixtures of olefines and 



oxygen corresponding to C„H„„-f-02 on explosion yield 



mainly carbonic oxide and hydrogen, without separation 

 of carbon, mixtures of paraffin and oxygen corresponding 



to C„n2n+o-l--02 yield carbon, oxides of carbon, methane, 



2 

 hydrogen, and steam, all in considerable quantities. Are 

 we then to conclude that there is some peculiarity about 

 the constitution of an olefine which induces a preferential 

 burning of its carbon, whilst the corresponding paraffin is 

 burnt in an entirely different way? The following experi- 

 ment will show that such a view cannot for a moment t>e 

 entertained. 



Experiment III. — I will now fire a bulb containing a 

 mixture of 60 per cent, of.ethvlene. and 40 per cent, of 

 oxygi-n (i.e. 3C,H,-(- 2O,). As might be expected, the 

 flame is accompanied by a large deposition of carbon, but 

 what is of greater importance still is the fact that a con- 

 siderable amount of w.tter is also formed. The full 

 significance of this experiment may be gathered from the 

 following data : — 



Original (C.jHjS 59-65 per cent. /, = 562mm. f.Jfii = l'4S 

 mixture \ '0.. = 40'35 ,, J>.2 = Si6 ,, 



Gaseous / CO.", = 2-5, CO = 37'2, C2H., + C2H4 = 6-4, CH4 = 6-5, 

 products! H„ = 47-4 per cent. 



C H O 



Units in original mixture ... 670 



Units in gaseous products ... 4S2 



670 

 572 



98 



227 

 172 



ss 



I think it will be now admitted that such an experiment 

 as this completely destroys the foundations of the theory 

 of the preferential burning of carbon. As I have already 

 stated, the original experimental basis of the theory was 

 the behaviour of an equimolecular mixture of ethylene and 

 oxygen, yet here is proof that on closer examination the 

 behaviour of ethylene is inconsistent with the theory, 

 which must, therefore, be abandoned. 



Experiment IV. — The next experiment is designed to 

 illustrate the infinitely greater affinity of acetylene and 

 ethylene as compared with that of hydrogen for oxygen 

 at the high temperatures of flames. I have here two bulbs 

 containing mixtures of each of these hydrocarbons with 

 hydrogen and oxygen corresponding to C^H^ + aH.-l-Oj 

 and C,H,, + H,-(-d, respectively, and I will ask you to 

 contras't the behaviour of these with that of the equi- 

 molecular mixture of ethane and oxygen, C.Ht-l-Oj, which 

 was exploded a few minutes ago. It should be noted that 

 whilst all three mixtures contain the same relative pro- 

 portions of carbon, hydrogen, and oxygen, they differ in 

 respect of the proportions between the combined carbon 

 and hvdrogen. Asking you to bear in mind how the 

 equimtilccular mixture of ethane and oxygen on explosion 

 gave rise to a black cloud of carbon and a considerable 

 formation of water, I will now fire the other two mixtures. 

 You will observe that in neither case has there been any 

 deposition of carbon, and an inspection of the cold bulbs 

 will show that little or no steam formation has occurred. 

 In fact, the hvdrocarbon has been burnt to carbonic oxide 

 and hydrogen,' leaving the hydrogen absolutely untouched 

 bv the oxvgen. 



'These experiments have an important bearing on the 

 chemistry of flames. Hydrogen is usually considered as 

 one of the most combustible of gases, but here we see it 

 pushed to one side by the all-powerful hydrocarbon _ as 

 though it were so much inert nitrogen. This at once raises 

 another question which has lately been occuoying my 

 attention. Ever since Davy's experiments on flame, the 

 combustibilitv of hydrogen has been considered to be 

 superior to that of' methane ; this, however, cannot be 

 true in regard to slow combustion, for it can be easily 

 proved that between 300° C. and 400° C. methane is 

 oxidised at a far faster rate than hydroffen ' in the absence 

 of surfaces, such as platinum or palladium, which readily 

 occlude hydrogen. 



It does' not, I think, impose too great a strain on the 

 imagination to picture the probable mechanism of com- 

 bustion in hvdrocarbon flaines, and for this purpose ethylene 

 and ethane mav be taken as tvpical examples. It may be 

 assumed that the aflinitv of a hydrocarbon for oxygen is 

 so great at high temperatures that the initial stage of its 

 combustion takes precedence of all other chemical pheno- 

 mena in flames. This is probably true of the propagation 

 of flame through explosive mixtures of hydrocarbons and 

 oxygen. In the special case of a stream of a hydrocarbon 

 burning in air, partial decomposition may occur in the 

 innermost regions of the flame, where the supply of oxygen 

 is verv limited, before combustion begins ; but, in general, 

 whenever the hydrocarbon and oxygen are brought together 

 at high temperatures, their mutual aflrinities will prove 

 superior to anv disruptive forces which would otherwise 

 break down the hvdrocarbon. It is probablv not so much 

 the original hydrocarbon as its hydroxylated mtjlecule 

 which decoinpos'cs in flaines ; the sudden increase in the 

 internal energv of the hydrocarbon molecule, consequent 

 upon its initial association with oxygen, would render the 

 "-esullini: hvdroxvlated molecule extremelv unstable, and. 

 in default of its immediate further oxidation, it would 



1 Since th- above was wr'tten. it has Veen prov.-d exnTiTienf-iHv 'hat 

 even in exp'osive rombnsiinn at high initial pressures the affinityof meth ine 

 greatly exceeds that of hydrogen for oxygen 



.VO. 2055, VOL. 80] 



