August 21, 1914 THE COLLIERY GUARDIAN 41 Explosive Combustion and Combustion by Incandescence. By J. MEUNIER.* Mixtures of gas and air may be divided into three classes : those containing an insufficient amount of combustible matter to be inflammable, inflammable and explosive mixtures, and extinguishing mixtures, which are too rich in gas and too poor in oxygen to support combustion. On the basis of these extinguishing mixtures it is possible to construct practical inflammability curves. For instance, take the case of coal gas, which, as sup- plied in Paris, ignites at the lower limit of 10 per cent., and forms an extinguishing mixture when the limit of 33 per cent is attained, by drawing from the abscissa 33 (fig. 1) a straight line at an angle of 45 degs. with the axis, and then setting up a perpendicular from the point 18 (which corresponds to the mixture giving per- fect combustion) until it meets the first line at A, junction of A with the point B (equal to 10 per cent., or lower limit of inflammability), we have the triangle ABC. By rounding off the apex of the triangle, at A —corresponding to the maximum velocity—since in the vicinity of this maximum the differences between the velocities are small, the inflammability curve, fig. 2, is obtained. Inner Zone of a Flame. — These principles may be applied to the study of flames. Taking first a simple flame, like that of a candle, the central zone round the wick is non-luminous, and there is no combustion there. This circumstance preserves the wick, which is merely © Percentage of gat Fig. 1. / j 5 10 15 70 «\W W Fig. 2. 20 30\*0 50 charred by the heat of the flame expelling the volatile constituents of the latter. This central zone, or base, of the flame contains the vapours of stearic acid, which are transformed progressively into gas by the heat, and can be extracted from the zone by aspiration. Luminous Zone.—The lower part of the base is sur- rounded by a blue stratum, which rises (tapering the while) to about the middle of the flame, the whole being surmounted by the white luminous flame. The blue stratum plays a very important part, which may be investigated by means of a batswing burner. It forms the outer luminous zone. The features of the candle flame are reproduced in that of the batswing burner, and the relative chemical composition of the various zones can easily be ascer- tained by means of the inflammability curve. In the dark inner zone the mixture is of the extinguishing type, whilst in the blue luminous zone the composition is that of an inflammable explosive mixture, and in the white flame more complex phenomena, which will be dealt with later, occur. Mechanical Phenomena of the Luminous Zone. —Ignition takes place in the zone of contact with the air, where the proportion of gas is at least 10 per cent., and is generally more. The mixture of air and gas is not, as believed hitherto, a simple physical mixture, the attraction of the air and gas producing an effect of shock. This important fact can be demonstrated by allowing a small test flame to impinge on the flame of the batswing burner. At the base of the blue luminous Fig. 3. zone the flow of air to the flame is at right angles to the latter, so that the test flame when approached to the nozzle of the burner, assumes a horizontal position (fig. 3); but as it is raised towards the upper part of the flame it gradually becomes vertical, and at the white zone is simply parallel to the main flame. At the base of a batswing flame there appear a number of filamentous corpuscles, which penetrate into the flame and at once become incandescent. These corpuscles do not originate in the gas, but in the sur- rounding air, and their yellowish luminosity is due to sodium and calcium. They consist chiefly of tiny particles of linen or cotton fibres, detached from cloth- ing, etc. Now, if these corpuscles be examined attentively, they will be found to enter the flame at right angles, but remain in the luminous zone. They thus indicate the path traversed by the air feeding the flame. Vortical Phenomena in Flames.—The attraction of the luminous zone on the surrounding air, coupled with the ascending movement of the hot gases, sets up vortical movements, and produces curved surfaces. * Bulletin de la Socidtd de I'Industrie Mindrale. Thus, as shown in fig. 4, a very decided and persistent helicoidal surface is produced on the flame of a bats- wing burner if the test flame already mentioned be brought close to it at a certain level. On one side of Fig. 4. the flame the twist is right handed, and on the other left handed; and the pitch of the spiral is a very slow one, the ascending velocity of the gas being high in proportion to that of the vapours of the test (spirit) lamp. Analogous curves can be observed in flames travers- ing flues, flames from wood fires, etc., as also the flames of the sun (as photographed by means of the spectroheliograph) and nebulae. Blowpipe Flames.—The introduction of air into the interior of a flame increases its temperature and activity chiefly through changing the composition of the mix- ture and rendering it more immediately combustible. At first the proportion of gas is 100 per cent., but the introduction of air quickly reduces that proportion. At the same time the external air contributes its contingent on the outside, so that blowpipe and similar flames are composed of two inflammatory movements, which meet together, and are represented by the two branches of the inflammability curve. The Bunsen burner forms the best apparatus for studying the variations of flames. When the air intake is closed the gas flame is white, and the dark zone extends about half-way up; but as the air intake is progressively opened the flame turns blue, with a white tip, and decreases in length, although the volume and pressure of the gas remain unchanged. The combus- tion is therefore more rapid. The dark centre assumes a decidedly conical shape. Interior Luminous Zone.—With a 40 per cent, mix- Fig. 5. E ture, which gives the best results so far as heating is concerned, the Bunsen flame is blue, with a small white tip. In the interior, above the base of the flame, is a very short zone of deeper blue. On admitting more air, so as to reduce the gas to 33 per cent., the white portion disappears entirely, and the total height of the flame is considerably lessened. The form of the basal zone S (fig. 5) changes to a decided cone, and its colour is of a deeper blue than that of the flame proper. The two outer blue bands E, which start from the orifice of the burner, elongated and spread out in the upper part of the flame, so that the latter is wholly blue, with a slight bulge on the level of the apex of the central cone. The velocity of combustion is more rapid than in the other cases, and the flame is therefore steadier. The central zone is not yet directly inflammable, but becomes so on the proportion of air being increased ever so slightly. On admitting more air, so that the central zone con- tains 27-28 per cent, of gas, the inner cone decreases to about IJin., instead of 3 in., in height, and the envelope I of the cone—internal zone of ignition—is a bluish green, the main flame being much broader than hitherto at the tip of the cone, and presenting the appearance of an inverted truncated cone surmounted by a right cone. Molecular Impact of Gas and Air : Swan Spectrum. —The inner and outer zones of luminosity are the seat of the molecular impact between the gas and air, and both of them give the particular spectrum, discovered by Wollaston, and more completely investigated by Swan, after whom it is named. This spectrum (com- prising five principal bands : orange, yellow, green, blue, and violet, arranged in harmonic series) is given by no other part of the flame, and is more intense in the inner zone than in the outer one. It is best observed by regulating the air supply so as to give a mixture with about 26 per cent, of gas; and its inten- sity forms a measure of the intensity of combustion. Variations of the Inner Luminous Zone.—It has been seen that the height of this zone (I) decreases as the proportion of air is increased. Commencing at 30 per cent, of gas, it exhibits another phenomenon, the length fluctuating (dancing) continually by several millimetres. This movement of elongation is the index of “ explo- sive combustion,” and the author has succeeded in transforming it into a succession of rapid explosions. So long as the basal zone S contains more than 33 per cent, of gas, the flame rests on a solid, because incom- bustible, base; but with smaller proportions (directly inflammable) it becomes unstable, and the flame cannot maintain itself unless the mixture of gas and air issues from the burner with a velocity exceeding that of ignition, which latter—as is indicated by the inflam- mability curve—however, increases as the proportion of gas recedes. Explosive Flames.—If a glass chimney be placed on the Bunsen burner (1, fig. 6) and a light applied, the . flame burns at the top of the chimney so long as too much air is not admitted; but as the air intake is opened, the flame sinks into the chimney and rises and falls, a series of explosive flames being formed, and - succeeding each other more rapidly in proportion as the air admission is increased. These flames are formed of green waves when the gas percentage is about 28-29 per cent., but blue at 24-26 per cent. The rapidity of succession also depends on the gas pressure, being about 300 explosions per minute with a pressure of 3£ in. water gauge, but 2-3 times as numerous with a pressure of 12-16 in. With a T-tube (2, fig. 6) the gas, lighted at the one open end, gives an explosive flame which is propagated, with a whirling motion, to the other end, where it will ignite combustible matter, gas jets, etc. (4, fig. 6). With an elbow tube (3, fig. 6) the flame is propagated in the same way as in a straight tube. If the tube be lengthened sufficiently at the opposite end to that at which the gas is lighted, the flame does not reach as far as the air, but terminates in a bluish tongue. The flame in the long tube, and that issuing from the short tube, both give the Swan spectrum, but the violet flame at the lighted end does not. This spectrum is, therefore, characteristic of the explosive flame, and results from the molecular impact of the gas and air. Phenomena of Luminosity.—Both the luminosity and stability of the flame are dependent on the velocity of inflammation. Several curious features will be men- tioned in dealing with convergent combustion; but it may be pointed out that Lemaire, in testing safety lamps in a current of firedamp, frequently observed the appearance of pointed flames which issued through cracks in the lamp glasses without igniting the gaseous mixture. This he ascribed to the mass of mixture ignited by these flames being too minute for any propagation of the ignition; but the author regards it as due to the difference in the velocity of the two gaseous masses, that of the issuing flame being much less than that of the current of firedamp. That ignition will not take place in these circumstances, though it does under converse conditions, is shown by directing a jet of gas, issuing from a blowpipe under a pressure of about 3 in. water gauge, against a gas flame of considerably lower pressure. The small jet does not become ignited, but traverses the flame and produces a dark cavity therein; but on the velocity of the jet being reduced by any means, ignition takes place at once. Convergent Combustion.—This is the term preferred by the author to express the phenomenon usually known as surface combustion, in which the gas mass ignites only on contact with a solid body, which it renders incandescent, the strata of gas converging towards the focus of combustion. If a platinum wire be inserted into a glass tube placed over a Bunsen burner, the wire becomes incandescent from the heat of the non-luminous flame; but on diminishing the supply of air, the incandescence decreases and finally ceases, though it reappears on the air supply being restored, provided it has not been allowed to cool down too long. The same property is acquired by copper, as the result of protracted heating, the metal remaining incandescent in a current contain- ing 30 per cent, of gas and losing its glow when the air intake is closed, the proportion of gas then becoming too large. These observations show that the inflammable strata converge on the incandescent solid and give rise to phenomena, not only of luminosity, but also of heat, which latter could not be produced if the gaseous strata in direct contact alone underwent combustion. That is to say, the combustion extends over a certain thick- ness of the gaseous strata. The following two rules may be formulated :—(1) The more explosive the mixture, the greater its activity in respect of convergent combustion; (2) the finer the filament the greater its luminosity. The nature of the filament must also be taken into consideration, the degree of convergent combustion produced being an inherent specific property of each substance, and depending also on the condition of its surface. These two rules underlie the action of the incandescent gas mantle, the luminosity of which increases with the explosibility of the gas mixture and the fineness of the filaments, increase of pressure being also an influential factor. Transition of Convergent Combustion into Explosive Combustion: Incandescent Lighting.—Convergent com- bustion or incandescence is characterised spectroscopi- cally by a continuous spectrum without bands. This combustion, however, is an unstable phenomenon, and any cause tending to disturb the supply of gas tends also to produce explosions or flame, more particularly at very high temperatures, and when the mass of the incandescent solid is small in proportion to that of the gas. If the mantle of a Welsbach burner be replaced by a platinum spiral, this latter becomes incandescent when heated by the flame, but ceases to glow when the supply of gas is checked. On gradually increasing the