Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/294

 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 wick, 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 nre 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, olefiant gas, carbon mon oxide, carbon dioxide, nitrogen in large proportion, aud small quantities of substances condensable to solids or liquids. The Kama gaseous constituents were found by Lindolt 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 ftames 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. &quot; Just before the disappearance of the yellow portion of the flame, there conies into view a splendid halo of pinkish light,&quot; due probably to incandescent nitrogen, &quot; forming a shell half an inch thick around the blue-green nucleus, and thus greatly enlarging the dimensions of the flame &quot; (Fraukland). 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 is 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 K&amp;gt; and 7 682 Ib 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 Journ. C/tem. Soc., March 1878, p. 9i). 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, &amp;lt;?.//., 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