Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/295

 FLAME 283 affinity of oxygen 1 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 names at high temperatures does not exist, and that at least for ordinary flames Davy s theory of the pro duction of luminosity holds good, &quot; 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&quot; (see Archives des 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 Journ. Tract. Chem., new series, vol. viii. p. 402). Hcumann 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 lias proved) is not altogether transparent, the appearance of a continuous image when an object is viewed through the flame Leiug 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 1 Compare the action of palladium sponge and foil on various hydrocarbon flames, probably through occlusion of hydrogen. See Wohlcr, in Phil. Mag., 1877, p. 35. the ratio of 100 to 4G 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. 1 1 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., Ixvii. 1 089. ) 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 flume, 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. (Frankland). The non-lumin osity of the flame of the Bunsen lamp, by means of which a mixture of coal-gas with 2 to 2i times its bulk of air is burnt, has been ascribed by some to the rapid destruction of the illumiuants 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. AVibel (Dent. Chem. Ges. JBer., 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 (J. Pract. Chem., is. 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 plungsd 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 iu which the flame is burning.