i6o 



NATURE 



[October 7, 19 15 



very narrow range, which is, within the limits of ex- 

 perimental error, the same for both air and oxygen 

 {i.e. in the case of hydrogen it is 580° to 590°, and 

 for carbon monoxide 640° to 658°). On the other 

 hand, in cases where the mechanism of combustion 

 is known to be very complex (i.e. hydrocarbons), the 

 ignition range is either fairly wide or else is materially 

 lower in oxygen than in air (or both). Thus — 



In air In oxygen 



Methane 65o°-75o° ... S56°-7oo° 



The explanation of such behaviour is probably to 

 be sought in the known complexity of the combustion, 

 and the marked tendency for appreciable and fairly 

 rapid interaction between the inflammable gas and 

 oxygen before the actual ignition-point is reached. 

 If, by any means, such preliminary interaction could 

 either be entirely suppressed, or if, on the other hand, 

 it be very rapid in character, the observed "ignition 

 range " would be narrowed, as is actually the case 

 with ethylene (542° to 547° in air, and 500° to 519° 

 in oxygen). 



There are two other means by which an explosive 

 mixture may be ignited. One is by adiabatic com- 

 pression, and the other, and most commonly employed 

 of all, is by the passage of an electric spark. The 

 adiabatic compression of an explosive mixture was 

 originally suggested by Nernst as a means of deter- 

 mining its ignition-point, provided (i) that ignition is 

 not produced locally, while the main temperature of 

 the gas is still far below the true ignition tempera- 

 ture ; (2) that the piston of the apparatus does not 

 move appreciably after the gas has been raised to its 

 ignition-point. At the time of my 19 10 report, Falk, 

 in America, had applied the method in the case of 

 hydrogen and oxygen mixtures, with results which, 

 in the light of more recent work, would appear to 

 have been misleading or erroneously interpreted. 

 Thus, for instance, he found that of all the mixtures 

 of_ hydrogen and oxygen, the equimolecular H. + O^ 

 mixture has the lowest ignition temperature (514°), 

 from which he concluded that the gases react initially 

 to produce hydrogen peroxide rather than steam. 

 Such a conclusion, which I believe to be erroneous, 

 naturf/ly directed attention to the experimental 

 method involved. 



The subject was promptly taken up here, in Man- 

 chester, by H. B. Dixon and his co-workers,* with 

 the result that much new light has been thrown on 

 the phenomena accompanying ignition. The ratio of 

 the ignition temperature to the initial temperature of 

 the mixture before compression, both expressed in 

 degrees absolute (T„/T,), may be calculated from the 

 compression ratio (V,/V,) by means of the well-known 

 formula for adiabatic compression : 



Ti 



'VJ- 



where 7 = the ratio of the specific heats at constant 

 pressure and volume, respectively, of the mixture com- 

 pressed, and which for a mixture of diatomic gases, 

 such as hydrogen and oxygen, is usually taken as 

 1-40. 



Dixon's recent photographic analysis of the appear- 

 ance of flame when mixtures of carbon bisulphide and 

 oxygen (CS2 + 3O0) are adiabetically compressed, have 

 proved that the flame, starting from a point or layer, 

 always takes an appreciable time to spread through 

 the mixture, and that unless special precautions are 

 taken to arrest the piston at the moment of its attain- 

 ment of the ignition condition, it may be driven in 

 much further than the minimum distance for ignition. 



4 H. B. Dixon, L. Bradshaw, and C. Campbell, Trans. Chem. Soc., 1914, 

 pp. 105, 2027 ; H. B. Dixon and J. M. Crofts, ibid., p. 2036. 



NO. 2397, VOL. 96] 



The real ignition-point, as above defined, is not neces- 

 sarily synchronous with the actual appearance of the 

 flame. There may be, and usually is, an appreciable 

 "pre-flame" period. Only in the fastest burning mix- 

 tures is this period negligible; hence the necessity of 

 artificially stopping the movement of the piston at the 

 beginning of the period — a precaution which Falk 

 seems to have neglected. 



According to Dixon and Croft's recent determina- 

 tion by this method of the ignition-points of mixtures 

 containing electrolytic gas, whereas successive addi- 

 tions of hydrogen or nitrogen progressively raise the 

 ignition temperature of the undiluted gas by regular 

 increments, as would be supposed, successive addi- 

 tions of oxygen, on the other hand, lower it, as a 

 glance at the following table will show : — 



Ignition Points of Mixtures Containing Electrolytic 

 Gas by Adiabatic Compression. 



(H. B. Dixon and J. M. Crofts, 1914.) 



Electrolytic Gas, 2H2+02 = 526° 



The observed raising effects of successive dilution? 

 with hydrogen and nitrogen call for no comment, save 

 that the relatively greater effect of hydrogen as com- 

 pared with nitrogen may be attributed to its greater 

 thermal conductivity; but the lowering effect of 

 oxygen is indeed puzzling, and its meaning can only 

 be conjectured. Dixon and Crofts have suggested 

 that it may be due either to the formation of some 

 active polymeride of oxygen under the experimental 

 conditions, which seems to me doubtful, or that the 

 concentration of oxygen in some way or other brings 

 about increased ionisation of the combustible gas. 

 This at once raises the larger question of whether or 

 not ignition is a purely thermal problem, as until 

 recently has generally been supposed. 



Prof. W. M. Thornton, of Newcastle-upon-Tyne, 

 recently published some very suggestive work on the 

 "Electrical Ignition of Gaseous mixtures,"* which, 

 apart from Its theoretical interest, has an Important 

 bearing on the safety of coal mines where electrical 

 currents are used for signalling and other purposes. 



The common belief that any visible spark will Ignite 

 a given explosive mixture of gas and air is, of course, 

 quite erroneous ; for just as Coward and his co. 

 workers have shown that for a given explosive mix- 

 ture and sparking arrangement there is a certain 

 limiting pressure of the gaseous mixture below which 

 ignition will not take place, so from Thornton's work 

 It would appear that a definite minimum of circuit 

 energy Is required before a given mixture at given 

 pressure can be ignited by a spark. And, moreover, 

 he has stated that the circuit energy required for the 

 spark ignition of a given mixture (say) of methane 

 and air Is something like fift}'-slx times greater with 

 alternating than with continuous current at the same 

 voltage. From this he has argued that the igniting 

 effect cannot be simply thermal, but must be in part 

 at least ionic This conclusion he further supports 

 with the statement that the igniting power of a uni- 

 directional current is proportional to the current in the 

 case of many gaseous mixtures over an important 

 part of their working range of inflammability. 



While there Is much that Is suggestive In Thorn- 

 ton's work, there Is also a good deal which seems 



5 Proc. Roy. Soc, A, vol. xc, 1914, p. 272: vol. xci., 1914, p. 17. 



