212 British Association.-Frog Bone's Address. taining electrolytic gas, whereas successive additions of (H. B. DIXON and J. M. CROFTS, 1914). = + xOg. xa 1 511° x- 7 478° x = 15 472° The observed raising effects of successive dilutions with hydrogen and nitrogen call for no comment, save that the relatively greater effect of hydrogen as compared 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 Crotts 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" (Proc. Roy. Soc., 1914, A, xc., 272; xci., 17), 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. This con. Influence of Electrons upon Combustion. During the discussion upon my 1910 report, Sir J. J. Thomson reminded us chemists that combustion 18 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 electrona 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 sur faces, 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 sup. posed rôle 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, negative results (Proc. Roy. Soc., 1914, A, xc., 506). 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 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 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 as yet to be no experimental grounds for attributing it to the ionising action of electrons 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 mixture 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, more over, he has stated that the circuit energy required for the spark ignition of a given mixture, say, of methane and air is something like 56 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. clusion he further supports with the statement that the igniting power of a unidirectional 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 Thornton's work, there is also a good deal which seems very difficult to interpret from a chemical standpoint. I here refer more particularly to his later supposition of "stepped ignition," which is based upon certain observed abrupt increases in the minimum igniting current required with condenser discharge sparks as the proportion of com. bustible gas 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-finally either dying out altogether or else giving rise to 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. The Initial Period of "Uniform Movement" or "Inflam mation" of Flame through 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 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, corresponding to the true rate of propagation "by conduction." This period of uniform movement is succeeded by an irregular oscillatory period, in which the flame springs backwards and forwards with increasing amplitudes detonation. With certain oxygen mixtures, the initial period of uniform slow velocity was shorter, and appeared to be abruptly succeeded by detonation without the inter vention of any oscillatory period. When, however, such mixtures were ignited near the closed end of a horizontal CHEMICAL NEWS, } Oct. 29, 1915 British Association.-Prof. Bone's Address. tube, the forward movement of the flame was continuously 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 was 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 abscissæ, they obtained "curves" which were in each case formed of two inclined straight lines converging 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 series of gas-air mixtures. Thus the curve they obtain for methane-air mixture was as Fig. 1, showing a maximum velocity of 61 cm. per second for a mixture containing about 12.2 per cent of methane, with lower and upper limits corresponding to 5.6 and 16.7 per cent of methane respectively. An exact knowledge of the velocities of flame propagation 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 propagation through such mixtures, not only at ordinary temperatures and pressures, but also when the mixtures are heated and used at higher pressures. Also, it would be important to know whether or not, in a 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 calculated from the known velocities for its single components. Unfortunately, although more than thirty years have elapsed since Mallard and Le Chatelier's work was published, the necessary data are still wanting to answer such questions, and anyone who will systematically tackle the problem aud carefully work it out in detail will be doing a real service to the gas-using industry. I am hoping shortly to make a beginning with such an investigation in my new department at the Imperial College, London; but the successful and rapid progress of such work will involve considerable financial outlay, as well as organisation and expert direction. Who will help us with the necessary funds? 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 clearer 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 Chatelier), 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 Moreover, suppress the influence of convection currents. 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 213 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 increased length probably makes no appreciable difference; 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 velocities 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 determining the composition of the upper and lower limits of inflammability by extrapolation from their curves has been proved to be unwarranted. Dr. Wheeler considers the length of the tubes used by Mallard and Le Chatelier (1 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 obviously be impure. The most important differences between the latest results published by Dr. Wheeler and those originally determined by Mallard and Le Chatelier, as shown on the accompanying diagram (Fig. 2), are as follows:1. 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 content respectively, whereas Mallard and Le Chatelier gave 5.6 and 16 7 per cent. 2. Whereas according to Mallard and Le Chatelier the flame velocities for mixtures near the upper and lower limits would gradually approximate to the zero velocity ordinate as the limiting composition was approached, according to Wheeler the velocities for both the upper and lower limiting mixtures are considerable (in each case about 36 cm. per second), and there is an abrupt change from these velocities to zero velocity as the particular limiting composition is passed. 3. Whereas Mallard and L Chatelier found a maximum velocity of 63 cm. per second for a mixture containing about 12.2 per cent of methane, and rapid falling-off in velocity as this particular composition is deviated from, Wheeler finds a maximum velocity of 110 to 112 cm. per second for a series of mixtures containing from 9:45 to 10:55 per cent of methane. Such differences as are thus disclosed only emphasise the need of a complete experimental revision of the subject. TABLE III.-Explosion of Mixtures of varying Composition between 2CH4 + O2 and CH4+ O2 in Bomb A at Initial Pressure 12.7 Atmospheres. than ordinary air. From their results (Table II.) it would appear that as the oxygen content of the atmosphere is reduced the limits of inflammability are narrowed until they coincide when the oxygen content falls below 13.3 per cent, which means that an atmosphere containing 13.3 or less per cent of oxygen is truly extinctive for a methane flame at ordinary pressures. 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 sion with sharp metallic click. No carbon deposited. 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 (Trans. Chem. Soc., 1914. cv., 1859). Their principal experiments were carried out in a rect angular 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 Fig. 5. 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, together with incompleteness of combustion, much of the combustible mixture remaining unburnt, and the question very naturally arises as to how the term "inflammability" 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 propagate flame indefinitely, the unburnt portion being maintained at that temperature and pressure. Inflammability thus defined would be a function of the temperature, 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 definition; also there is a possibility that in a mixture just at or very near one or 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 : Hydrogen Carbon monoxide Per cent. 4'I 5'3 (a) 12.6 (a) Too much stress need not be laid upon the differ. ence between this number and the 5.6 per cent given by Dr. Wheeler (loc. cit.), because Dr. Coward himself admits that the flames of mixtures con. taining from 5.3 to 5.6 per cent of methane 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. 216 British Association,-Prof Bone's Address. Combustion of Hydrocarbons and Relative Affinities of Methane, Hydrogen, and Carbon Monoxide respectively for Oxygen in Flames. Under the title of "Gaseous Combustion at High Pressure" I have recently published, in conjunction with various collaborators (Messi S. Hamilton Davies, H. H. Gray, H. H. Henstock, and J. B. Dawson), a further | instalment of my researches on the mechanism of hydrocarbon combustion (Phil. Trans. Koy. Soc., 1915, A, ccxv., 275), and I may, perhaps, be allowed to draw your attention to certain new points which have arisen in connection therewith. A detailed study of the behaviour of mixtures of methane and oxygen, of composition ranging between 2CH4+O2 and CH4+O2. when exploded in steel bombs at initial pressures of 12.7 atmospheres (see Table III.), has shown it to be consistent with the "hydroxylation" theory of hydrocarbon combustion which I put forward some years ago as the result of my previous work. The following scheme seems to interpret correctly the chemical and thermal changes involved in the initial stages of the explosive combustion of methane. CHEMICAL NEWS kilogrms. Centigrade units, whereas (2) of the same Again, if the original mixture contained only one-half such proportion of oxygen (2CH4+ O2), there would still be a decided preference for an oxidation of half of the methane by a non-stop run A to C through B rather than an oxidation of the whole of the methane to B only, the other half of the methane remaining unchanged or undergoing thermal decomposition into its elements, CH4 C+ 2H2 (A to A,); also the latter process would use up no more of the energy developed from the oxida tion than would be required for the decomposition of a corresponding quantity of H3:C.OH at stage B (B to B'). Hence, when such a mixture 2CH4 O2 is exploded under pressure the formation of carbon and its oxides, hydrogen, and considerable quantities of steam may be expected, which is precisely what actually occurs. When, however, the proportion of oxygen in the original mixture reaches the limit 3CH4+ 202, while it is still insufficient to oxidise the whole of the hydrocarbon to the dihydroxy stage, there is enough of it to prevent any methane remaining unoxidised to at least the monohydroxy stage, and therefore, seeing that the affinity of methane for oxygen far exceeds those of either hydrogen or carbon monoxide, it is to be expected that no substantial propor tion of the original methane would escape oxidation to either the mono or the dihydroxy stage. But inasmuch as not more than about one-third of the original methane The principal experimental facts which this-or, indeed, could in the circumstances be transformed into the diany alternative-scheme must explain are: 1. That whenever mixtures of composition between 2CH4 O2 and CH4 +0 are exploded under pressure, a considerable proportion of the original oxygen appears as steam in the products. 2. That there is a marked minimum in the proportion of such oxygen as the composition of the origina mixture approximates to 3CH4 + 202. 3. That there is a total cessation of any separation of carbon (which is very marked with mixtures 2CH4+02) after the proportion of oxygen in the original mixture attains or exceeds the limit 3CH4 + 202. Now if, as I believe, the initial interaction of methane and oxygen is at all temperatures essentially a "hydroxylation " process, accompanied by the decomposition (more or less rapid according to the temperature) of the primarily formed hydroxylated molecules, a consideration of the chemical and thermal aspects of the process will point tu certain possibilities which are, indeed, actually realised in fact. In the first place monohydroxymethane (methyl alcohol) CH3.OH is known to decompose at high temperature, yielding carbon monoxide and oxygen, without any separa or any separation of carbon formation of steam. CH3OH = CO + 2H2; also the very unstable dihydroxymethane, H2:C:(OH)2, would yield formaldehyde and steam, H2:C(OH)2 = H2:C:O + H2O, and the formaldehyde would in turn decompose into carbon monoxide and hydrogen, H2:C:O: CO + H2, without any deposition of carbon whatever. If now the thermal consequences of such facts be considered, it would annear (1) that were the oxidation of methane to H3:C.OH (A to B in the scheme) accompanied by thermal decomposition at this stage (B to B'). the net heat evolution would be (30-22.8) 72 hydroxy stage, it follows that a considerable amount of thermal decomposition at the monohydroxy stage would occur. If this view is correct it follows that there should be an entire surpression of carbon deposition at or about the 3CH4+202 ratio, and also that with this particular mixture a smaller proportion of the original oxygen should arpear as steam in the products than would be the case with either the 2CH4+ O2 or the CH4 O2 mixture, which again is precisely what we find. In considering the question of the explosive combustion of hydrocarbon, it is important to distinguish between (1) the primary oxidation of the hydrocarbon, which is an exceedingly rapid process, and is probably completed during the short interval between ignition and the attain ment of maximum pressure, and (2) certain probable secondary interactions where influence may extend far into the subsequent cooling period, for it is only this latter which would be affected by variations in the rate of cooling down from the maximum temperature. Such secondary interaction may include the reversible charge CO OH2 CO2 + H2, and in cases where carbon is derosited as the result of the decomposition of primary oxidation products the interaction of steam and carbon C+ OH2 = CO • H2. In this connection I may draw attention to the recently published work of G. W. Andrew (Trans. Chem. Soc., 1914, cv., 444), one of my former pupils and collaborators, on the "Water-Gas Equilibrium in Hydrocarbon Flames," which proves that in a system containing only CO2, CO, H2. H2O, rapidly cooling down from the high temperatures prevailing in hydrocarbon flames, the equilibrium ratio COOH adjusts itself automatically with the tempera CO2 x H2 ture until a point between 1500° and 1600° C. on the cooling curve is reached (corresponding to a value K=40 approximately), after which no further readjustment |