62 



NATURE 



[October 7, 19 15 



it out in detail will be doing a real service to the gas- 

 using industry. 



An accurate knowledge of the behaviour of methane- 

 air mixtures under known variations of conditions is 

 of prime importance from the point of view of the 

 safety of coal mines, and it is rightly occupying the 

 attention of my friend and former collaborator, Dr. 

 R. V. Wheeler, at the Home Office Experimental 

 Station at Eskmeals. From papers which he has 

 already published, as well as from some unpublished 

 results which he has very kindly permitted me to refer 

 to in this address, it is now possible to correct certain 

 errors in Mallard and Le Chatelier's results, and to 

 arrive at a clear view of the phenomena as a whole. 



In the first place, it would appear that the initial 

 "uniform movement" of flame in a gaseous explosion, 

 or, in other words, propagation of the flame from layer 

 to layer by conduction only (as defined by Le 

 Chateiier) is a limited phenomenon, and is only 

 obtained in tubes of somewhat small diameter — wide 

 enough, however, to prevent appreciable cooling of 

 the flame, but narrow enough to suppress the influ- 

 ence of convection currents. Moreover, ignition must 

 be either at or within one or two centimetres of the 

 end of the tube ; otherwise — particularly with the more 

 rapidly moving flames — vibrations may be set up right 

 from the beginning. 



While all methane-air mixtures develop an initial 

 uniform slow flame-movement period when ignited at, 

 or near, the open end of a horizontal tube, both its 

 linear duration as well as the flame velocity are not, 

 according to private information which Dr. Wheeler 

 has sent me, independent of the dimensions of the 

 tube. The speed of flame increases with the diameter 

 of the tube ; and the linear duration of the uniform 

 period increases with both the diameter and length of 

 the tube up to a certain maximum, after which in- 

 creased length probably makes no appreciable differ- 

 ence. Also, for the same tube, it varies with the 

 proportion of methane in the explosive mixture — being 

 greater as the speed of the flame diminishes, until 

 with the two "limiting" explosive mixtures it appears 

 to last almost indefinitely. 



Dr. Wheeler's recent re-determination of the velo- 

 cities of the flame movement during this initial uniform 

 period for mixtures of methane and air in varying 

 proportions within the limits of inflammability, has 

 revealed serious errors in Mallard and Le Chatelier's 

 original results for horizontal tubes of the same 

 diameter as those which Dr. Wheeler has employed. 

 Moreover, Mallard and Le Chatelier's method of deter- 

 mining the composition of the upper and lower limits 

 of inflammability by extrapolation from their curves 

 has been proved to be unwarranted. Dr. Wheeler con- 

 siders the length of the tubes used by Mallard and Le 

 Chateiier (i metre only) was insufficient to ensure that 

 the speed measurements of the initial uniform flame- 

 movement period were unaffected by the subsequent 

 "vibratory period." Also, the methane used by them, 

 prepared as it was from sodium acetate, would obvi- 

 ously be impure. According to Wheeler, the limits of 

 inflammability for horizontal propagation of flame in 

 methane-air mixtures, at atmospheric temperature and 

 pressure, correspond to 5-4 and 14-3 per cent, methane 

 contents, respectively. 



Messrs. Burgess and Wheeler have recently deter- 

 mined the limits of inflammability of methane when 

 mixed, at atmospheric temperature and pressure, with 

 "atmospheres" of oxygen and nitrogen containing 

 less oxygen than ordinary air. From their results 

 (see below) it would appear that, as the oxygen content 

 of the atmosphere Is reduced, the limits of Inflamma- 

 bility are narrowed until they coincide when the oxygen 

 content falls below 13-3 per cent., which means that 



NO. 2397, VOL. 96] 



an atmosphere containing 13-3 or less per cent, of 



oxygen Is truly extinctive for a methane flame at 

 ordinary pressures. 



Atmosphere Methane, per cent. 



Behaviour of Weak Mixtures of Gases and Air. 



My review of this part of the subject would be 

 incomplete without a reference to some interesting 

 observations which have been made by Dr. H. F. 

 Coward and co-workers at the Manchester School of 

 Technology, upon the behaviour of weak mixtures of 

 various inflammable gases and air, at, or just below, 

 the lower limit of Inflammability in each case." Their 

 principal experiments were carried out in a rectangular 

 box of 30 cm. square section and 1-8 metres length, 

 with two opposite sides of wood, and the other two 

 of plate glass. The box was placed in an upright 

 position, the bottom being water-sealed and the top 

 closed, with a suitable igniting device placed near the 

 bottom. They have shown that caps or vortex rings 

 of flame may be projected for some distance upwards 

 from the source of ignition — sometimes apparently for 

 an indefinite distance — without igniting the whole of 

 the combustible mixture. In such mixtures there may 

 be an indefinite upward slow propagation of flame, to- 

 gether with incompleteness of combustion, much of 

 the combustible mixture remaining unburnt, and the 

 question very naturally arises as to how the term "in- 

 flammability" should be scientifically defined. Dr. 

 Coward has argued, with some force, that a gaseous 

 mixture should not be termed " inflammable " at a 

 given temperature and pressure, unless it will propa- 

 gate flame indefinitely — the unburnt portion being 

 maintained at that temperature and pressure. Inflam- 

 mability thus defined would be a function of the tem- 

 perature, pressure, and composition of a particular 

 mixture only, and would be Independent of the shape 

 and size of the containing vessel ; and, provided that 

 It is kept in mind that for each particular mixture 

 at a given temperature and pressure a certain 

 minimum Igniting energy and intensity is requisite, I 

 am inclined to agree with the definlton. Also, there 

 is a possibility that In a mixture just at, or very near, 

 one or other of the limits of inflammability, flame may 

 be propagated upwards, but not downwards. 



From his experiments. Dr. Coward has assigned 

 the following as the lower limits of inflammability of 

 hydrogen, methane, and carbon monoxide respectively, 

 in air at atmospheric temperature and pressure : — 



Per cent. 



Hydrogen 4-1 



Methane 5-3' 



Carbon monoxide 12-6 



Recent Investigation upon the Combustion of Hydro- 

 carbons and the Relative Affinities of Methane, 

 Hydrogen, and Carbon Monoxide respectively for 

 Oxygen in Flames. 

 (This part of the address, which was read as a 

 separate paper, reviewed the principal results of an 

 Investigation by Prof. Bone and collaborators on 



' Trans. Chem. Soc, 1914, vol. cv., p. 1P59. 



8 Too much stress need not he laid upon the difference between this 

 number and the 5-6 per cent, given by Dr. Wheeler (loc. a'i.) because Dr. 

 Coward himself admits that the flames of mixtures containing from 5*3 t0 5'6 

 per cent, of me'hane are very sensitive to shock ; while a 5-6 per cent, 

 mixture will always propagate flame indefinitely, even when there is a 

 moderate disturbance. The conditions must be exceedingly tranquil to 

 prevent extinction in the other cases. 



