82 



NATURE 



[March i8, 1909 



" led to imagine " that it " might be owing to the decom- 

 position of part of the gas towards the interior of the 

 ilame where the air was in smallest quantity, and the 

 deposition of solid charcoal, which, first by its ignition 

 and afterwards by its combustion, increased in a high 

 degree the intensity of the light." It is important to 

 observe that not only did Davy rightly attribute the 

 luminosity of a hydrocarbon flame to the presence therein 

 of incandescent carbon, but also that he avoided the error 

 of attributing the separation of carbon to a supposed 

 preferential burning of hydrogen. 



In considering the propagation of a flame through an 

 explosive mixture of gases, it is necessary to distinguish 

 between two well-defined conditions. When such a mix- 

 ture is ignited, the flame travels for a certain limited 

 distance (a few feet only) at a fairly uniform slow velocity, 

 which in the case of a mi.xture of hydrogen and oxygen 

 in their combining ratios is approximately 34 metres 

 (38 yards) per second. This initial stage of the combus- 

 tion is called " inflammation." 



After traversing a few feel, however, the flame begins 

 to vibrate, and alters in character. The vibrations be- 

 come more and more intense, the flame swinging back- 

 wards and forwards with oscillations of increasing ampli- 

 tude. Then one or other of two things happens ; either 

 the flame is extinguished, or it goes forward with an 

 exceedingly great and constant velocity, producing the 

 most violent effects. The new condition thus set up is 

 termed "detonation," and the forward movement of the 

 fiame is called the '^explosion wave." 



The discovery of " detonation " in gaseous mixtures 

 •was made simultaneously by M. Berthelot and MM. Malard 

 and Le Chatelier in the year 1881 ; Berthelot proved that 

 the velocity of the explosion wave is independent of the 

 length of the column of gas traversed, and that for the 

 same gaseous mixture under given physical conditions it 

 always has a constant value. In this connection I must 

 mention Prof. H. B. Dixon's exhaustive researches on 

 the " rates of explosion " of gaseous mixtures, which have 

 extended in so many ways our knowledge of explosive 

 combustion. 



Experiment I. — Perhaps the best illustration of the out- 

 ward difference between ordinary " inflammation " and 

 ■" detonation " is afforded by the case of a mixture of 

 carbonic oxide and oxygen in their combining 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 mix- 

 ture is fired in a leaden coil, where the brief initial period 

 of inflammation is succeeded by the explosion wave, which 

 dashes onwards through the gases at a rate of 1700 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 experiinents carried out under Prof. Dixon's direc- 

 tion, it was found that the duration of luminosity in each 

 successive layer of gas in the detonation of electrolytic 

 gas does not exceed i /5000th 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 oxygon, has been the 

 subject of much discussion during the past fifteen years. 

 The main points in dispute may be conveniently sum- 

 marised under three heads. 



(i) During the greater part of last century the belief 

 prevailed that the hydrogen is much the more combustible 

 of the two elements of a hydrocarbon, and that conse- 

 quently when combustion occurs in a limited supplv of 

 oxygen, the hydrogen is preferentially burnt, as follows : — 

 C5Hi4-Oj,=2C + 2HjO. 



Who was the author of this view, or what was origin- 

 ally its experimental basis, is not quite clear, but it re- 

 ceived the active support of two such eminent authorities 

 as Thomas Graham and Michael Faraday, and for fifty 

 years it was regarded as one of the most certain articles 

 NO. 2055, VOL. 80] 



of chemical faith. It was finally overthrown by Dixon 

 and Smithells in the year 1892. 



(2) The second theory originated with Kersten in 1861, 

 who, as the outcome of experiments on the explosion of 

 a mixture of ethylene and electrolytic gas, asserted that 

 "before any portion of the hydrogen is burnt, all the 

 carbon is burnt to carbonic oxide, and that the excess of 

 oxygen then divides itself between the carbonic oxide and 

 the hydrogen." In other words, Kersten attempted to 

 substitute the idea of the preferential burning of carbon 

 for that of the preferential burning of hydrogen. His 

 views, however, received no serious attention until they 

 were revived bv Dixon and Smithells. 



The chief experimental basis for this theory is the 

 behaviour of ethylene and acetylene when exploded with 

 their own volume of oxygen. More than a century ago 

 Dalton found that a mixture of equal volumes of ethylene 

 and o.xygen yields mainly carbonic oxide and hydrogen on 

 explosion, without any separation of carbon, in conformity 

 with the equation 



C2H4 + OJ = 2CO-I-2H2 



I vol. I vol. 2 vols. 2 vols. 



This fact, after being overlooked for nearly eighty years, 

 was re-discovered by Di.xon in .i8gi ; moreover, a few 

 years later, when it was proved that acetylene behaves in 

 a precisely similar manner, 



QHj + O, = 2C0 4-H,, 



I vol. I vol, 2 vols. 1 vol. 



the advocates of the theory were able to claim a con- 

 siderable body of evidence in support of their case. 



(3) But the idea of a " preferential " combustion, 

 whether of carbon or of hydrogen, seemed repugnant to 

 well-established principles concerning the nature and con- 

 ditions of chemical interactions in gaseous systems. More- 

 over, whilst the assumption of a direct passage from an 

 initial system of ethylene and oxygen, C,H,, -l-Oj, to the 

 system carbonic oxide and hydrogen, 2CO + 2H,, implied 

 a simple transaction from the kinetic standpoint, an 

 extension of the idea to the case of such a hydrocarbon as 

 propylene, 



zCaHg + 30.J = 6CO -t- 6Hj, 



would at once raise serious difficulties. 



It therefore remained to consider whether the solution 

 of the problem might not lie in the assumption of an 

 initial association of the hydrocarbon and oxygen forming 

 an unstable " oxygenated " molecule, which subsequently 

 rapidly decomposes. Thus, for example, the changes in- 

 volved in the explosive combustion of an equimolecular 

 mixture of ethylene and oxygen might conceivably be re- 

 presented somewhat as follows : — 



C„Hi + 05 = [C2H4O.,] = 2CO + 2H2 

 unstable 



Many years ago, indeed, Prof. H. E. Armstrong sug- 

 gested that the combustion of a hydrocarbon takes place 

 under the conjoint influence of water and oxygen, and 

 involves the successive formation of intermediate 

 " hydroxyiated " molecules, which at high temperatures 

 rapidly decompose into simpler products. Little notice was 

 taken of his suggestion at the time, but recent researches 

 have shown that " hydroxyiated " molecules are probably 

 formed, even in flames, although I think it doubtful 

 whether water vapour is an essential factor in the process. 



The researches recently carried out at the Manchester 

 University have covered the entire range of conditions 

 under which hydrocarbons can be burned, from the slow, 

 flameless combustion discovered by Davy right up to the 

 extreme conditions of detonation. An exhaustive study of 

 the slow combustion of methane, ethane, ethylene, and 

 acetylene, at temperatures between 250° C. and 400° C, 

 afforded decisive evidence against the preferential burn- 

 ing, whether of carbon or of hydrogen. Large quantities 

 of aldchydic intermediate products were isolated, and the 

 balance of evidence was decidedly in favour of the 

 " hvrfro.vv/a'Voii " Iheorv. with the proviso, however, that 

 the oxygen is directly active. 



