Flame, the luminous appearance caused by the combustion of gases or vapors. When a liquid or solid is burned so as to form a flame, it is first converted into gas or vapor. The small blue flame which appears upon burning charcoal is caused by the union of atmospheric oxygen with the carbonic oxide gas which is the first product of the union of oxygen with carbon. The structure of a flame is best observed in the burning of a sperm or tallow candle, or an oil lamp having a solid wick. In the candle flame, represented in section in fig. 1, the central dark inner cone a, surrounding the wick and proceeding to a point a short distance above it, is chiefly composed of light and heavy carburetted hydrogen gases, formed by the action of heat on the melted fat, and such as are contained in common illuminating gas, of nitrogen obtained from the air, of watery vapor, and also of carbonic oxide and carbonic acid gases. In the blue zone, b, at the base of the flame, the gas of the base of the inner cone is completely burned by oxygen less rarefied than that which reaches other parts of the flame. This zone has the same character as the inner flame of the blowpipe. That part of the flame which furnishes the principal part of its light is called the luminous cone, represented at c.
Its base surrounds the inner cone, its apex reaching above it. It is luminous in consequence of the incandescence of numerous minute particles of solid carbon which have been formed by the abstraction of the constituent hydrogen of the carbo-hydro-gen gas, and its union with atmospheric oxygen. The supply of oxygen to the inner parts of this flame is not sufficient to consume the carbon, but the combustion of hydrogen furnishes sufficient heat to produce white light in the particles of solid carbon. The combustion is completed in the outer cone, d, by the union of carbon and remaining unconsumed gases, with atmospheric oxygen. It is called the mantle, and is much less luminous than the cone last described, the light being principally caused by incandescent gas and vapor. This part of the flame is sometimes confounded with the blue zone at the base, but the mistake can readily be demonstrated by holding a piece of cardboard between the eye and the flame in such a way as to cut off the luminous cone, and thus enable the difference in the character of the light of the two cones in question to be distinguished. The flame of a lamp wick is of course similar to that of a candle; and a flat flame has the same structure, only the part corresponding to the inner cone is very thin.
The blue-colored parts of an ordinary flame are chiefly owing to the combustion of carbonic oxide gas, which may be considered as the transition state of the carbon element during its complete union with atmospheric oxygen and formation of carbonic acid. The upward current of heated gas produced by a flame burning in air, undisturbed by external currents, is quite rapid, as may be observed when smoke is allowed to be carried up with it, and the resistance which it offers to horizontal currents is considerable.-The structure and composition of a candle flame may be demonstrated in several ways. If a fine metallic wire is passed horizontally through the centre, it will soon become incandescent in the luminous cone and mantle on each side of the flame, while that part which is in the dark inner cone will slowly become only slightly red from conduction of heat, or not at all if the wire is very small. If the wire is of steel or iron, after being held for some time in the flame it will be found on examination to have become corroded at those points which were in the mantle and outer part of the luminous cone, in consequence of combining with atmospheric oxygen, which it readily does under the influence of incandescent heat.
That part of the wire which is in the inner cone will not be affected, while that part which is in the inner part of the luminous cone will be covered with lampblack. If a silver or copper wire which has had the surface tarnished by oxygen be employed, the tarnish will disappear at those points which are in the inner part of the luminous flame, because the oxygen which had united with the metal is now taken up by the heated free carbon in the flame. In the outer cone or mantle the coating of oxide will be increased. This furnishes an explanation of the nature of the common or mouth blowpipe flame, which is produced by blowing a fine stream of air through the flame of an oil or alcohol lamp or a candle, fig. 2. The tip of the blowpipe is usually introduced into the inner cone, and air from the mouth is forced through it, which mingles atmospheric oxygen with the combustible gases, and produces complete combustion of all those portions in the line of the jet. The whole flame is directed by the current of the jet, and a current of air surrounding it also passes in the same direction.
The whole of the gases of the inner cone are not consumed by the air blown through the pipe (unless it be too large), but a portion is left to be consumed in the luminous outer hollow cone, c, where it meets with the oxygen of the air. As the oxides of metals are reduced to a metallic state by parting with oxygen to the carbon, the inner flame a is called the reducing flame, and its point b is also the hottest point in the whole flame. The outer cone is the oxidizing flame, which varies in quality in different portions, the most effective point for most purposes of oxidation being at the tip, although the flame is used in a variety of ways, depending upon the material under examination and the nature of the substance in which it is held. If a piece of fine wire gauze is held in a horizontal position and lowered into the flame, the latter will only continue to burn below it, the unconsumed gases passing through, but without sufficient heat to burn. A central dark circle, a section of the inner cone, will then be observed, and also a luminous outer ring, formed of the luminous cone and the mantle. (See fig. 3.) If, while the gauze is held in the flame, a lighted taper be applied to the upper surface, the unconsumed gas will take fire, and the original flame will be nearly restored, the gauze forming a horizontal section.