284 FLAME The decree 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 illnminants, 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 substano 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 iu 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. Rossetti (Journ. dc 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 G50. 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 lessening 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 2| 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; arid 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 I Ol mm. and minimum 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 iu 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 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, which 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. May. [4], xxxiv. p. 489, 1867.)
