PRESIDENTIAL ADDRESS. 371 



in the air mixture examined progressively increases. In other words, it is 

 claimed that continuous alteration of the proportions of gas and air in an 

 explosive mixture is, or may be, accompanied by discontinuous alterations in 

 the spark energy required for ignition. I must confess that, after careful 

 examination of the published curves, I am quite at a loss to give them any 

 chemical intei-pretation, and to being somewhat sceptical about this supposed 

 ' stepped ignition.' 



A repetition and extension of Professor Thornton's experiments would be 

 most valuable as a means to a better understanding of the conditions of spark- 

 ignition. 



The Influence of Electrons upon Combustion. 



During the discussion upon my 1910 report, Sir J. J. Thomson reminded 

 chemists that combustion is concerned not only with atoms and molecules but 

 also with electrons moving with very high velocities. They might be a factor 

 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 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 molecules in the act of combining sending out electrons with 

 exceedingly high velocities, which precede the explosion-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 

 University upon the combination of hydrogen and oxygen in contact with a 

 gold surface," which lend some support 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. 



Professor H. B. Dixon and his pupils have, at Sir J. J. Thomson's sug- 

 gestion, recently tested the idea as applied to the explosion-wave, with, however, 

 negative 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 question which Professor 

 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 

 efTect upon either the initial phase of an explosion or upon the high velocity 

 of detonation. But although he employed a very intense magnetic field, pro- 

 duced by 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 explosion. And inasmuch as the high constant velocity of the explosion 

 wave can be entirely accounted for on the theory of a compression-wave 

 liberating the chemical energy as it passes through the gases, there seem to be 

 as yet no experimental grounds for attributing it to the ionising action of 

 electrons. 



The Initial Period of ' Uniform Movement of Flame ' thro^igh Inflammable 



Mixtures, and Limits of Inflammability. 



Mallard and Le Chatelier, in their classical researches 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 



• Proc. Poy. Soc. 1914. 



' Proc. Boy. Soc. 1914, Sec. A, vol. 90, p. 506. 

 • Ann. des Mincx, 188.3 (A) 4. 274. 



B B 2 



