FLAME, the phenomenon exhibited when gas or a gaseous mixture is actively undergoing combustion. Solid bodies (such as coke) which do not contain inflammable volatile constituents, that is, constituents which become gaseous and ignite upon the application of heat, become in candescent when they burn, but their combus tion is not attended by the development of flame. The flames that appear to issue from such solid combustibles as wood and coal are in reality attendant upon the combustion of gases distilled from the fuel by the heat, as may be very easily proved by subjecting these bodies to the same temperature in closed vessels which prevent the access of air. Abundant quantities of combustible gases are then given off and these may be led away in pipes or stored in tanks, to be burned at more convenient times or places. The gas obtained from coal in this way has been used for many years for lighting through the luminous flame with which it burns. Flames are usually very hot and luminous, though in some cases there is but little light developed. The flame of an alcohol lamp, for example, is barely visible, although it is intensely hot ; and the flame of burning hydrogen is an even more striking case of the same sort., (See also PHOSPHORESCENCE) . The luminosity of flames is due in some cases to the incandescence of tiny particles of solid carbonaceous matter that they hold in suspension; in the majority of instances, however, the luminosity of the flame is due to incandescence of some gas at a very high temperature. Experiments have demon strated that luminosity of a flame depends on the density of the gases which are present in the flame and that flames which are but feebly luminous in an atmosphere of normal pressure become luminous when the burning gases are compressed. Even the hydrocarbon flames owe their luminosity not to the solid particles of car bon present, but to the glow of dense hydro carbon vapors. A pertinent instance is that of the contrary phenomenon of the loss of luminosity of a candle flame when placed in a situation where the normal atmospheric pres sure upon it can be considerably reduced.
In studying the action which takes place in a flame the round type has been found simplest in structure — that is, the flame issuing from a round pipe. The flame in such case is conical. This is due to the fact that the cylinder of flowing gas burns only on the outside — where it is in contact with the air. The gas on the inner section of the flame has to travel farther before it meets the air, and the central part of the flowgets to the farthest point. The flame
then is a hollow cone of combustion, filled with gas which has not yet burned because it is not in contact with oxygen. It is the incandescence of this Inner gas which produces the light. On the other hand it is the outer shell of actual combustion which produces the heat. It is a common experiment in physics to conduct the gases from the interior of the cone of a candle flame by means of a small glass tube to a dis tance of a few inches away from the flame and to ignite them as they issue from the tube.
When a piece of wire gauze is brought down over the flame of a candle„ the gauze permits the gases that arise from the wick to pass through it, but it conducts away their heat so that they are no longer capable of combining with one another and hence the flame does not extend above the gauze. That the gases developed at the wick really rise up through the gauze is easily proved by igniting them with a burning match. A certain degree of heat is needed to cause gas to ignite and burst into flame, but with most inflammable gases, once they are ignited the heat evolved in their com bustion is sufficient to continue the condition of flame. The ignition points of some of the Com moner combustible gases are as follows — in degrees Fahrenheit: hydrogen sulphide, 685°; acetylene, 800'; ethylene, 1,000'; ethane, 1,060'; hydrogen, 1,085'; carbon monoxide, 1,200°; methane, 1,300° ; cyanogen, 1,550°. The temper atures of some of the flames used in the indus tries as sources of heat are: acetylene, 3,114°; alcohol vapor (with air), 3,384°; coal gas with air—in the Bunsen burner, 3,400° ; hydrogen, with air—in the Bunsen burner, 3,452'; oxy gen-coal gas blowpipe, 3,992°; oxyhydi ogen blowpipe, 4,388° ; alcohol blowpipe, 4,618'; oxy acetylene blowpipe (about), 5,700°. See also Flames may be colored, and often very bril liantly, by the presence within them of certain metallic salts. Thus sodium compounds pro duce a strong orange-yellow color, potassium a violet and strontium a crimson. Advantage is taken of this fact for the production of gorgeous color effects in pyrotechny (q.v.). In chemical analysis, too, the color that a given substance yields when placed in a hot but normally non luminous flame often gives valuable information concerning its composition. See BLOW-PIPE ANALYSIS; SPECTROSCOPE.