October 7, 1915] 



NATURE 



161 



-TV difficult to interpret from a chemical viewpoint. 



I here refer more particularly to his later supposition 



if "stepped ignition," which is based upon certain 



bserved abrupt increases in the minimum igniting. 



urrent required with condenser discharge sparks as 



he proportion of combustible gas in the air mixture 



xamined progressively increases. In other w'ords, it 



- claimed that continuous alteration of the propor- 



ions of gas and air in an explosive mixture is, or 



!iay be, accompanied by discontinuous alterations in 



he spark energy required for ignition. I must con- 



■ss that, after careful examination of the published 



urves, I am quite at a loss to give them any chemical 



interpretation, and to being somewhat sceptical about 



the supposed "stepped ignition." A repetition and 



extension of Prof. Thornton's experiments would be 



most valuable as a means to a better understanding 



of the conditions of spark ignition. 



Influence of Electrons upon Combustion. 



During the discussion upon my igio report, Sir 

 J. J. Thomson reminded us chemists that combustion 

 is concerned, not only with atoms and molecules, but 

 also with electrons moving with very high velocities. 

 They might be a fact of prime importance in such 

 intensive forms of gaseous combustion as are realised 

 in contact with hot or incandescent surfaces, as also 

 in the explosion wave. It is well known, of course, 

 that incandescent surfaces emit enormous streams of 

 electrons travelling with high velocities, and the 

 actions of such surfaces may be due to the formation 

 of layers of electrified gas in which chemical changes 

 proceed with extraordinarily high velocities. Again, 

 the rapidity of combustion in the explosion wave 

 might, he thought, conceivably be due to the mole- 

 cules in the act of combining sending out electrons 

 with exceedingly high velocities, which precede the 

 e.xplosion wave and prepare the way for it by ionising 

 the gas. 



With regard to this interpretation of the action of 

 surfaces, Mr. Harold Hartley carried out a promising 

 series of experiments in my laboratory at Leeds Uni- 

 versity upon the combination of hydrogen and oxygen 

 in contact with a gold surface, which lend some sup- 

 port to the idea. But they require further extension 

 before it can be considered as finally proved. It is 

 my intention in the near future to resume the 

 systematic investigation of the matter as rapidly as 

 circumstances permit ; but the experimental difficulties 

 are formidable, and the mere chemist working by 

 himself may easily be misled. We badly need the 

 active co-operation of physicists in elucidating the 

 supposed role of electrons in combustion. 



Prof. H. B. Dixon and his pupils have, at Sir J. J. 

 Thomson's suggestion, recently tested the idea as 

 applied to the explosion wave, with, however, nega- 

 tive results.* It is known, of course, that the motion 

 of the ions can be stopped at once by means of a 

 transverse magnetic field, in which they curl up, and 

 are caused to revolve in small circles ; and the ques- 

 tion which Prof. Dixon decided to put to the test of 

 experiment was whether the damping of the electronic 

 velocities in a powerful magnetic field would have 

 any appreciable effect either upon the initial phase of 

 an explosion or upon the high velocity of detonation. 

 But though he employed a very intense magnetic 

 field produced by some powerful magnets specially 

 constructed by Sir Ernest Rutherford for the deflection 

 of electrons of high velocity, no appreciable effect 

 was observed upon the character or velocity of the 

 flame with any gas mixture at any stage of the ex- 

 plosion. And inasmuch as the high constant velocity 

 of the explosion wave can be entirely accounted for 



I* Pirc. Roy. Soc., 1914, Section A, vol. xc, p. 506. 

 NO. 2397, VOL. 96] 



on the theory of a compression wave liberating the 

 chemical energy as it passes through the gases, there 

 seem as yet to be no experimental grounds for 

 attributing it to the ionising action of electrons. 



The Initial Period of " Uniform Movement " or 

 ^'Inflammation" of Flame through Inflammable 

 . Mixtures, and Limits of Inflammability. 



Mallard and Le Chatelier, in their classical re- 

 searches upon the combustion of explosive mixtures, 

 discovered that the propagation of flame, when such 

 a mixture is ignited in a horizontal tube, differs 

 according as whether the ignition occurs near the 

 open or closed end of the tube. In the first case, the 

 flame proceeded for some distance down the tube at 

 a practically uniform and fairly slow velocity, corre- 

 sponding to the true rate of propagation "by con- 

 duction." This period of uniform movement is suc- 

 ceeded by an irregular oscillatory period, in which 

 the flame swings backwards and forwards with in- 

 creasing amplitudes, finally either dying out altogether 

 or else giving rise to detonation. With certain 

 oxygen mixtures, the initial period of uniform slow 

 velocity was shorter, and appeared to be abruptly 

 succeeded by detonation without the intervention of 

 any oscillatory period. When, however, such mix- 

 tures were ignited near the closed end of a horizontal 

 tube, the forward movement of the flame was con- 

 tinuously accelerated from the beginning, under the 

 influence of reflected compression waves, until 

 detonation was set up. Such, in general, was the 

 sequence of the phenomena that were observed by 

 these distinguished French investigators. 



They proceeded to determine experimentally the 

 velocities of the uniform slow movement of the flame 

 in the case of a number of air and combustible gas 

 mixtures, and plotting their results (in cms. per sec.) 

 as ordinates against percentages of inflammable gas 

 as abscissae, they obtained "curves" which were in 

 each case formed of two inclined straight lines con- 

 verging upwards to a point which represented the 

 composition and flame velocity of the most explosive 

 mixture. And they concluded that the points at which 

 the downward production of the two lines met the 

 zero velocity line would define the upper and lower 

 limits of inflammability for the particular scries of 

 gas-air mixtures. Thus the curve they obtain for 

 methane-air mixtures showed a maximum velocity of 

 61 cms. per second for a mixture containing about 

 12-2 per cent, of methane, with lower and upper limits 

 corresponding to 56 and 167 per cent, of methane 

 respectively. 



An exact knowledge of the velocities of flame pro- 

 pagation during this initial period of uniform slow- 

 movement, as well as of the limits of inflammability 

 for mixtures of various combustible gases and air, is 

 very important from a practical point of view. 

 Makers of apparatus for burning explosive mixtures 

 of gas and air want to know the speed of flame pro- 

 pagation through such mixtures, not only at ordinary 

 temperatures and pressures, but also when the mix- 

 tures are heated and used at higher pressures. Also, 

 it would be important to know whether or not, in 

 the case of a complex mixture of various combustible 

 gases and air, when complete composition can be 

 determined by analysis (as, for example, coal gas and 

 air), the velocity of flame propagation can be calcu- 

 lated from the known velocities for its single com- 

 ponents. Unfortunately, although more than thirty 

 years have elapsed since Mallard and Le Chatelier 's 

 work was published, the necessary data are still want- 

 ing to answer such questions ; and anyone who wilf 

 sj'stematically tackle the problem and carefully work 



