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Flame




FLAME. An ordinary flame consists of a gas or vapour burning in contact with the air, and in most cases emitting light of greater or less intensity. It is thus distinguishable from the mere incandescence or glow produced by a burn-ing body which does not become gaseous previous to com-bustion. Many solid substances, however, burn with flame, because under the influence of heat they either, as wood and coal, evolve combustible gases and vapours, or, as zinc, sulphur, and phosphorus, become volatile. Charcoal, although a solid, when brightly heated in a furnace yields flame, since carbon dioxide, or carbonic acid gas, in pass-ing through it, furnishes the combustible gas carbon mon-oxide, or carbonic oxide. "When the air or other supporter of combustion is intimately mixed with the combustible gas, ignition occasions an explosion, and the flame resulting is instantaneous and equally luminous throughout. The dark inner cone of a candle-flame (see BLOWPIPE, vol. iii. p. 837, col. 2) is formed by the volatilized tallow raised by the capillary action of the wdck, together with carbonic oxide, carbonic acid gas, and water vapour from the com-bustion going on in the outer part of the flame, and atmospheric nitrogen, but no free oxygen. By means of a short tube inserted into it, the inner cone of the flame may be made to yield up a portion of its contents ; the applica-tion of a light to the other end of the tube shows that they are combustible. That within the flame they are not ignited may be demonstrated in many ways. Thus, if a piece of paper be depressed upon the flame for a few seconds, that part of it which touches the central part of the flame is not charred. Again when a platinum wire is held horizontally in a candle flame, it is heated to redness at the two parts where it is in contact with the outer zones of combustion, remaining dark between them. In the intermediate, white-hot, and luminous zone of the candle flame the oxidation of the gases yielded by the inner core is chiefly effected. The reducing effect of the flame is greatest at the surface of contact of these two portions. On account of the slower combustion, the smaller propor-tion of unoxidized gases, and the greater penetration of oxygen at the upper part of the flame, the luminous zone is thicker, but less bright above than below. In the outer non-luminous zone or mantle, which envelops the whole flame except at its base, combustion is completed. The mantle may be conveniently observed by interposing a small piece of card between the luminous cone and the eye. At the base of the flame, reaching a little upward towards the inner cone, is a light blue zone.

The composition of wax and tallow flames, according to Hilgard, is hydrogen, marsh-gas, defiant gas, carbon mon-oxide, carbon dioxide, nitrogen in large proportion, and small quantities of substances condensable to solids or liquids. The same gaseous constituents were found by Landolt in coal-gas, with tetrylene, water, and a very little oxygen. The size of a flame is in relation to the amount of oxygen required to consume a definite bulk of its con-stituent gas, and to the purity of the oxygen supplied. Hence the flames produced by the same bulk of different gases vary considerably in magnitude, the flame of hydrogen being smaller than that of olefiant gas, but larger than that of a mixture of hydrogen and nitrogen. The flame of oxygen burning in marsh-gas, of which it needs only half its volume for complete combination, is much smaller than that of oxygen burning in hydrogen, of which latter two volumes are required. The tapering shape of a candle-flame results from the spreading of the gases set free in its interior, from the ascent of the products of combustion, owing to their having a higher temperature and consequently a lower density than air, from the currents in the cooler atmosphere around it thereby occasioned, and, lastly, from the exhaustion of combustible material at the upper part of the flame. A flame of large size tends to be irregular in form through variations in the force of air-currents. By altering the atmospheric pressure to which they are sub-jected, flames may be made to differ considerably in shape. Thus at two atmospheres of pressure the flame of a sperm candle is spike-like, and scarcely one quarter of an inch in diameter below, whilst its upper part is enveloped in smoke, in which the apex is concealed. When the normal pressure is gradually reduced to that of half an atmosphere, the flame is changed in appearance chiefly through diminu-tion of its luminosity; but as the pressure is lowered thence to that of six inches of mercury, the flame becomes globular, and assumes a greenish-blue tint. " Just before the disappearance of the yellow portion of the flame, there comes into view a splendid halo of pinkish light," due probably to incandescent nitrogen, " forming a shell half an inch thick around the blue-green nucleus, and thus greatly enlarging the dimensions of the flame " (Frankland). The colour of flames, due, according to Heumann, to heated vapours, is dependent principally on their temperature and on the nature of bodies contained in them. The flame of carbonic oxide, ordinarily blue, is yellowish-red if the gas be heated before ignition. The colour of flames is subject also to modification according as one or other of any two gases burnt together i3 made the supporter of combus-tion.

Flames may be classed as luminous, such, e.g., as those produced by the burning of wax, tallow, oils, and other carbonaceous substances, and metals ; and non-luminous, as those of hydrogen, alcohol, sulphur, and carbon monoxide. A good illuminating flame may readily be procured from a non-illuminant gas by saturating it with the vapour of a heavy hydrocarbon ; thus hydrogen and marsh gas, when burnt with one pound of benzol, give a light equal to that yielded by 5-793 lb and 7-682 lb of spermaceti respectively. The diluents of coal-gas, namely hydrogen, carbon mon-oxide, and marsh gas, afford practically no light; the light given by the illuminants is not, however, altogether independent of the relative proportions of the diluents (Frankland andThorne, in Joum. Chem. Soc., March 1878, p. 94). Flames transparent for all lights are non-luminous, since the power of an incandescent gas at a given tempera-ture to absorb and its power to emit light are proportional to each other.

Sir H. Davy, from experimental investigations, was led to the conclusion that the luminosity of flames is caused by minute solid particles of incandescent carbon or other sub-stances set free from the combustible body by heat. From researches by Frankland and other physicists it appears that this view is not tenable in respect to all flames. The flame of arsenic burning in oxygen emits a remarkably intense white light, although neither the metal nor the product of its combustion, arsenious acid, is gaseous at the temperature of the flame. Again, carbon disulphide and nitric oxide give on combustion together a light which is almost unbearable by the eye, and which, like that of a coal-gas flame, affords a perfectly continuous spectrum, and yet no solid particles are concerned in its production. From these and similar facts, and from the seeming trans-parency of illuminating flames, it has been concluded by Frankland and others that although a non-luminous flame may be rendered luminous by the introduction into it of substances in the solid condition, e.g., asbestos, platinum wire, and fine powders, the light-giving power of ordinary flames is dependent to a great extent on the density of their constituent gases and vapours, and not on the presence in them of any solid particles. Further, the deposits of soot formed by the flames of ordinary illuminants on bodies with which they come in contact have been regarded, as consisting, not of solid carbon set free by the elective affinity of oxygen for the hydrogen of the hydrocarbon, but of mixtures of condensed hydrocarbons of remarkably high boiling-points. The observations of several experi-menters do not, however, bear out these conclusions. M. J. L. Soret has demonstrated that the supposed transparency of many flames at high temperatures does not exist, and that at least for ordinary flames Davy's theory of the production of luminosity holds good,—" a pencil of solar light being reflected by diffusion and polarized in precisely the same manner, whether it falls on a very brilliant flame, or whether it illuminates non-incandescent smoke, in which the presence of carbon particles is incontestable" (see Archives ties Science, July 1874, trans, in Phil. Mag., 1875). Stein, moreover, has proved that the sooty deposit obtain-able from a coal-gas flame contains not more than 9 per cent, of hydrogen, and that, were it merely condensed vapour, exposure to a high temperature would cause its volatilization, which, however, is not the case (see Jmrn. Pract. Chern., new series, vol. viii. p. 402). Heumann holds that for the production of light from a hydrocarbon flame a high temperature is requisite,—first, to set free solid particles of carbon, and, secondly, to maintain these in a state of incandescence. In support of the theory that the luminosity of hydrocarbon flames does result from the existence within them of carbon particles he points out—

(1) That the luminous mantle of a hydrocarbon flame (as Stein also has proved) is not altogether transparent, the appearance of a continuous image when an object is viewed through the flame being attributable probably to the smallness of the illuminating particles and their rapid motion ;
(2) That flames the luminosity of which is due to the presence of finely divided solid matter give shadows when viewed in sunlight; also that luminous hydrocarbon flames give shadows ; and that the only light-giving flames that are shadowless are those consisting of glowing gases and vapours ;
(3) That chlorine increases the luminosity of feebly luminous hydrocarbon flames by setting free in them particles of what must certainly be pure carbon ;
(4) That the presence of solid particles in a hydrocarbon flame may be demonstrated by causing it to impinge upon a heated surface, or upon a similar flame; and
(5) That the lower surface of a small rod placed in a flame becomes covered with soot, which has been separated in the lower portion of the flame. This deposit cannot be the result of the cooling action of the rod, as it is not formed on all sides of the rod, and may be produced on hot as well as cold bodies introduced into the flame.





The influence of pressure upon luminosity has next to be considered. It was shown by Davy in 1817 that the intensity of the light of flames is increased by the condens-ation, and diminished by the rarefaction of the atmosphere. Frankland has proved that flames ordinarily non-luminous, as those of hydrogen, carbonic oxide, and alcohol, can be made highly luminous by condensing the atmosphere in which they burn. Hydrogen yielding, under a pressure of three atmospheres, light estimated at one unit, at twelve atmospheres gives a light of 100 units. On the other hand the flame of arsenic in oxygen loses greatly in lumin-osity when subjected to reduced pressure. For each decre-ment of pressure from 30 down to 14 inches of mercury, according to Frankland, the flames of candles undergo an equal or nearly equal loss of luminosity ; for lower pres-sures the diminution is less rapid. The blue portion of the flames of candles burnt on the summit of Mont Blanc was found by the same experimenter to extend to the height of one eighth of an inch above the cotton, the rate of combus-tion being the same as at Chamounix; and it has been computed by him that, owing to the difference of barome-tric pressure in the two cities, the illuminating effects of the same sample of coal-gas in London and Mexico must be in the ratio of 100 to 46-2. From the above-mentioned facts the inference has been drawn that the decrease in light caused by rarefaction of air is attributable to the greater mobility at low pressures, and consequent readier oxidation of gaseous bodies, and also to the increase in the size of the flame. Conversely, the greater luminosity under pressure has been ascribed to the augmentation of the, density of the burning gas or vapour thereby occasioned. It has, however, been proved that, although the density of constituents is not without effect on the luminosity of flames, it cannot be con-sidered alone as a cause of this phenomenon, since at high pressures the temperature of a flame is increased. (See Deville, Compt. Rend., lxvii. 1089.) Now, although the light and heat of flames are not related in degree, for the flame of the oxyhydrogen lamp, the hottest known, is scarcely luminous, yet for the same kind of flame luminosity increases with temperature.

The influence of temperature on luminosity may be exemplified in various ways. Thus, if the cooling of a flame be checked by decreasing its surface in respect of its volume, a greater amount of light is obtained. The smoky flame of turpentine becomes luminous when its tempera-ture is raised ; and if, by the use of an outer glass cylinder in addition to an ordinary lamp chimney, the air supply-ing a flame be Warmed at the expense of the escaping pro-ducts of combustion, a considerable increase of light is the result. Heumann finds that the cooling effect of a metallic burner on a gas flame notably diminishes its luminosity ; and he attributes the greater intensity of the light observed when the burner is heated to an earlier separation of carbon particles in the flame, and to their more vivid incan-descence.

Considerable insight into the conditions of the luminosity of flame has been afforded by experiments with mixtures of illuminating gas and air. The light of pure coal-gas being reckoned at 100 units, that of gas with 10 per cent, of air is 33 units, with 20 per cent. 7, with 30 per cent. 2, and with 40 per cent. 0 (Frankland). The non-lumin-osity of the flame of the Bunsen lamp, by means of which a mixture of coal-gas with 2 to 2-J- times its bulk of air is burnt, has been ascribed by some to the rapid destruction of the illuminants of the gas by the oxygen of the admixed air; but some further explanation of the phenomenon is evidently necessary, since the volume of air employed is inadequate for the complete oxidation of the gas, and the flame is still lightless if hydrogen, carbonic oxide, steam, or indifferent gases, as nitrogen, carbon dioxide, or hydro-chloric acid, be substituted for air. Wibel (Beut. Chern. Ges. Ber., viii. 226), finding that the heating of the gas and air previous to ignition renders the flame luminous, concludes that the non-luminosity of the flame is the result of the cooling action of the gases entering it; but, as Heumann points out, it cannot be wholly due to this cause, for the temperature of the non-luminous flame of the Bunsen burner, as also of the blowpipe, is much above that of ordinary flames. From the observations of Stein (/. Pract. Chern., ix. 183), who shows that a flame made non-luminous by nitrogen is yet hot enough to decompose coal-gas, and that carbon monoxide, which has a pyrometric effect nearly equal to that of coal-gas, renders the flame of that gas non-luminous without lowering its temperature, it is evident that the mere introduction of a diluent into a burning gas diminishes its light, independently of any absorption of heat to which it may give rise. Heumann finds that decrease of luminosity through dilution and cool-ing takes place when a gas-flame giving light in ordinary air is plunged into a mixture of five volumes of air with two volumes of carbon dioxide, or when the products of combustion are allowed to accumulate in the air in which the flame is burning.

The degree of rapidity with which oxidation takes place is a condition which affects very considerably the lumin-osity of flames. If a small flame, free from unoxidizable substances, be placed in pure oxygen, its light becomes feeble, since the illuminants, instead of spreading through it in an incandescent state, are speedily oxidized ; within certain limits, therefore, the mixture with the oxygen of an inert gas, e.g., nitrogen or carbon dioxide, increases the luminosity of the flame. The simplest form of gas-burner, having a single orifice only, affords the minimum amount of light, as the gas rushes without interruption into the air. When, however, as by the use of the fishtail burner, two jets of gas are made to impinge upon each other, the velocity with which the illuminants are driven through the flame and oxidized is retarded, with the effect of con-siderably augmenting the light. The insertion of a small piece of platinum plate between the two jets, as in Scholl's "platinum perfecter," by reducing still more their velocity, causes a further increase in the luminosity of the flame (Frankland).

The temperature of flames differs considerably according to the conditions of combustion and the nature of the substance burnt. That of hydrogen in air, calculated from its absolute thermal effect as measured by Favre and Silbermann, is 2080° C; that of carbon monoxide, 2828°; and that of marsh-gas, 1935°. The oxyhydrogen flame lias a temperature estimated by Bunsen at 2844° C. The flame of a common candle in its hottest portion is at a temperature high enough to melt a small filament of platinum held in it. Deville, experimenting with a mix-ture of oxygen and carbon monoxide, found that, at 54 millimetres above the orifice of the burner employed, the flame was hot enough to melt gold; at 12 mm. it melted platinum; and at about 2 mm. lower, at the apex of the inner cone of the flame or a little beneath it, the highest temperature was indicated. The temperature of the flame of alcohol, according to Becquerel, is nearly 1204° C. (2200° Fahr.). M. F. Bossetti (Journ, de Phys., vii. 61), by means of a thermo-electric element of iron and platinum, estimated the temperature of the external envelope of a Bunsen flame at 1350° C, that of the violet portion at 1250°, and that of the blue at 1200°, the temperature in the central dark cone ranging from 250° to 650°. The flame of a mixture of two volumes of illuminating gas with three of carbon monoxide indicated a temperature of 1000°. The determination of the higher temperatures of flame by means of thermo-electricity is, however, open to considerable sources of error.

The nature and the continuance of the combustion of a flame depend (1) on the supply of the supporter of the combustion; (2) on the ignition-temperature of the gas or vapour; and (3) on the heat produced in burning, and therefore on the degree of rarefaction of the atmosphere, which by lesseniug chemical combination diminishes the heat of the flame.
(1) In the case of all ordinary flames the oxygen of the air is the supporter of combustion, and of this a free supply is requisite. A combustible hydrocarbon containing more than six parts of carbon to one of hydrogen burns with a smoky flame in air, unless a free draught can be provided by the use of a lamp-glass. A coal-gas flame sup-plied by an orifice of one quarter of an inch in diameter is not smokeless when higher than 2J inches; but the flame is rendered clear if by lengthening or dividing the aperture of the jet the exposure of a larger extent of its surface to the air is effected.





(2) Davy (Phil. Trans., 1816, pt. i. p. 117) points out that a large quantity of air thrown upon a small flame lowers its heat below the exploding point of its con-stituents, and that the extinction of a flame by blowing upon it is probably produced by that cause, assisted by the dilution of the explosive mixture. The fact that, by the presence or neighbourhood of a cooler body, the temperature of a heated gas or vapour is lowered beneath the igniting point, explains the action of the wire gauze of the Davy lamp (see COAL, vol, vi. p. 72), and also explains why a candle-flame does not quite touch its wick, or a gas-flame its burner, unless the latter be some-what strongly heated. The presence of an inactive gas hinders, and if in large quantity prevents, by its cooling action, the explosion of gases. The rate of propagation of ignition in gaseous mixtures thus varies not only with the nature of their combustible constituents, but with the degree to which these are diluted with indifferent gases. The rate in a mixture of hydrogen and oxygen in com-bining proportions is 34 millimetres a second; and that in a similar mixture of carbonic oxide and oxygen is less than 1 mm. a second (see Bunsen in Pogg. Ann., cxxxi. 165). The maximum velocity of the propagation of igni-tion for marsh-gas and air, according to M. E. Mallard (Ann. des Mines, lii. 355, 1875), is '524 mm., the minimum _041 mm. a second; the velocities of coal-gas and air are maximum V01 mm. and minimum 0'097 mm. a second. A jet of combustible gas at high pressure, or much diluted with inert gases, can be ignited only at a considerable distance from the orifice at which it issues, owing apparently to the cooling action of the gas itself and of the outer air, and perhaps more especially to the velocity of the gas being greater than that of the propagation of ignition within it (Heumann).

(3) From numerous experiments, chiefly with gases, Davy concluded, first, that the extinction of flame on rare-faction of the atmosphere takes place only when the heat produced by the burning body is insufficient to keep up the combustion, the mere removal of pressure from the burn-ing body being without effect on its combustibility; and secondly, that therefore those bodies which require least heat for their combustion burn in more rarefied air than those requiring more heat, and those which produce much heat in their combustion, other circumstances being the same, burn in more rarefied air than those producing little heat (Phil. Trans., 1817). Hence the extinction of a gas by rarefac-tion is hindered by raising its temperature. Davy found that a mixture of oxygen and hydrogen in combining pro-portions could not be exploded when rarefied to one eighteenth of its normal density, and that hydrogen would not burn at a pressure of one-seventh of an atmosphere. Compression tends to make the combustion of flames less perfect, so that flames smoky at ordinary pressure can be rendered smokeless by rarefying the atmosphere in which they are burning.

It has been shown by Professor R. Bunsen that the combustion of a uniform mixture of an inflammable gas with oxygen takes place discontinuously. Thus, when a mixture of carbon monoxide and oxygen in combining proportions is exploded in a closed vessel, its temperature rises from 0° to 3033° C., and two-thirds of the carbon monoxide remains unconsumed and incombustible until, by radi-ation and conduction, the temperature sinks to 2558° C. Below this temperature a second burning begins, whicli restores the temperature to 2558° C., at which point it abides stationary until exactly half the carbon monoxide is consumed. The inflamed mixture now cools to 1146° C, when the combustion ceases ; it must, however, be afterwards continued at lower as at higher temperatures, since the product of combustion consists eventually, on cooling, of carbon dioxide only. Bunsen has further pointed out that there is a simple molecular relation between the quantities of compounds which, in favourable circumstances, are formed simultaneously in a perfectly uniform gaseous mixture ; as, for example, when water and carbon dioxide are produced from hydrogen and carbon monoxide burnt with less oxygen than suffices for the combustion of both. (Of. CHEMISTRY, vol. v. p. 483, col. 2.) This relation undergoes sud-den alterations by the addition, by degrees, of a third body, as oxygen, the homogeneous nature of the mixture not being affected. (See Phil. Mag. [4], xxxiv. p. 489, 1867.)

The radiation of heat from the flame of a Bunsen burner is considerably less than that from a luminous gas flame, and by most vapours is less powerfully absorbed. The absorption by the air of the heat radiated from a hydrogen flame, the source of which is the aqueous vapour produced by the combustion, has been shown by Tyndall to be due to the presence of moisture in the air, dry air being actually transparent to the radiations of the flame, whereas on humid days undried air may absorb as much as 20-3 per cent, of them, This phenomenon is explicable on the theory that, notwithstanding the high temperature of the flame, there is an accord between its oscillating molecules and those of aqueous vapour at the ordinary temperature, the heat of the flame increasing the amplitude but not the rate of its molecular vibration. When a carbonic oxide flame, the product of which is carbonic acid gas, is made to radiate through an atmosphere of the same gas, the absorption is very great, insomuch that, at a pressure of 4 inches, 65 per cent, of the radiation is cut off. The radiation of heat from a flame, it is thus apparent, depends on the length of the heat waves to which, accord-ing to the nature of its products, it gives rise, and on the character of the atmosphere through which the radiations have to pass. The products of the combustion of alcohol are aqueous vapour and carbonic acid gas, the heat rays of which have a slow period of vibration and correspond to the ultra-red rays of the spectrum. The temperature of an alcohol flame may be lowered by plunging into it a spiral of platinum wire, but its heat, being thereby con-verted into heat of higher refrangibility, is consequently more readily transmitted through some substances, such as glass, than that of the original flame. (See Tyndall, Heat as a Mode of Motion, 5th ed., 1875, p. 385 sq.)

The phenomena of singing flames have already been alluded to (ACOUSTICS, vol. i. p. 115). M. C. Decharme has produced persistent and varied sounds by directing a jet of air from a tube with a diameter of 3 to 5 mm. on a gas-flame supplied by an orifice of similar diameter. The effect of the jet of air seems to be attributable in great measure to its chemical besides its mechanical action. By altering the diameter and position of the tubes, and the nature and pressure of the gas and air, the sounds and the colour and shape of the flame may be greatly modified, (Compt. Rend., lxxx. p. 1602.) On the spectra of flames see SPECTRUM ANALYSIS.

In consequence of the rarefaction of the air which it occasions, and also its tapering form, flame acts with great rapidity in dissipating a charge of electricity. It has been shown by Grove (Phil. Mag., 1854, [4], vii. 47) that a current of electricity is transmitted in flame, and is produced in it probably by chemical action.

Dr Edmund Hoppe, in a paper on the electrical resist-ance of flames (Nachrichten v. d. K. Ges. d. Wiss. u. d. Georg-Augusts-Universitdt, 1877, p. 313), concludes as the result of his experimental investigations that—

(1) For every flame the electric conductivity increases with an increase of the heat and of the amount of the burning gases; (2) The relative conductivity of different flames is dependent on the nature of the substances burnt; the vapours of salts and solu-tions are particularly efficient in augmenting the conductivity of the flame of hydrogen; and (3) Ohm's law, contrary to the surmise of Hankel (Aba. d. Kbnigl. Sticks. Ges. d. Wiss., v. p. 72, 1861) is applicable to the case of flames.

The diamagnetism of flame, on its discovery by M. P. Bancalari, was in 1847 investigated both by Zantedeschi and Faraday. When placed in various positions between the poles of a powerful electro-magnet, the flame of a wax taper becomes inclined, or assumes a fishtail shape, or even spreads out right and left in an equatorial direction between the poles, producing a double flame with two long tongues. Faraday found that when a small flame only about one third of an inch high was used, the magnetic force flattened it into an equatorial disc. The brightest flames appeared to him to be the mostdiamagnetic (Faraday, Exp. Researches, vol. iii. pp. 467, 487, 490).

See, in addition to the authorities quoted above, Works of Sir H. Davy, ed. by Dr John Davy, vol. vi. pp. 1-130, 1840; Frankland, A Course of Lectures on Coal-Gas, 1867, and Experimental Researches, 1877; and Dr Karl Heumann, Contributions to the Theory of Luminous Flames, translated by M. M. Pattison Muir from Liebig's Ann. der Chemie, vol. clxxxi. pt. 2, pp. 129-153, and vol. clxxxii. pp. 1-29, in Phil. Mag., 1877, pp. 1, 98, 366. On the transparency of coloured flames see Gouy, Compt. Rend., lxxxvi. 878-880. See also GAS. (F. H. B.)


Footnotes

Compare the action of palladium sponge and foil on various hydrocarbon flames, probably through occlusion of hydrogen. See Wohler. in Phil. Mag., 1877, p. 35.




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