Flame consists of gaseous matter undergoing combustion. In this process heat is given out as the result of the chemical combinations effected, and also, generally, light from the incandescence of some of the substances undergoing combustion.
When the gas and the supporter of its combustion (i.e. the substance with which it combines in burning), which is usually oxygen or atmospheric air, are thoroughly mixed together, and when any part of it is raised, as by the application of a light, to the temperature of ignition, combination is effected with great rapidity throughout the mass of the gas, and a flame of no particular shape or form is produced. But if the combustible gas issues from a jet or orifice and is burned there, the flame assumes a definite shape, and exhibits certain more or less well-defined parts. The flame of such a substance as coal-gas issuing from a jet consists of three parts:
(1) An inner dark space filled with the gas alone, and in which no combustion takes place; it may be termed the area of non-combustion, and that it is of this nature may be proved by inserting into it a small piece of phosphorus, when it will be found that the phosphorus does not take fire, as it would were it placed in any other part of the flame.
(2) A luminous envelope, completely surrounding the dark inner part except at the jet, and consisting of the combustible gas mixed with oxygen and atmospheric air. Into this part of the flame the oxygen of the air penetrates and effects combustion; but, as it is not present in sufficient quantity to completely burn all the constituents of the gas, part of the carbon is separated; this portion of the flame is therefore termed the area of partial combustion.
(3) An outer portion in which the separated carbon particles are raised to incandescence and completely burned, as also are any other products the combustion of which has not been completely effected in the area of partial combustion; hence this is known as the area of complete combustion.
The colour of a flame depends partly on its temperature, but principally on the nature of the substances undergoing combustion or incandescence. The luminosity of flames varies also with the nature of the gas; it is due to the incandescence of carbon particles, and also of dense hydrocarbons present. Feebly luminous flames, such as those of hydrogen, can be increased in luminosity by the admixture with the combustible gas of such substances as benzole vapour; also by an increase in pressure of the agent supporting the process of combustion, or by heating the gas before it enters the flame. Of 100 units of light emitted by a gas flame burning under an atmospheric pressure of 30 inches of mercury, 5.1 units are lost by each reduction in pressure of 1 inch.
Singing and Sensitive Flames.—If the flame of coal-gas or, better still, hydrogen issuing from a blowpipe-nozzle be slowly passed up a wide glass tube, it will be found that at a particular position up the tube the flame alters its character; being rapidly extinguished and rekindled, while at the same time it gives forth a peculiar musical note. To this is given the name of a singing flame. The sound is caused by longitudinal vibrations of the air in the tube, originally set up by a decrease in pressure above the flame, caused by the upward draught; a downflow is thus momentarily produced, which in its turn is checked by the upward motion of the air; and so on.
When an inflammable gas issues from a narrow vertical jet, and is ignited above a sheet of wire-gauze held horizontally about two inches above the orifice, a flame is produced which is easily affected by any sound-waves which strike it, and is hence termed a sensitive flame. Such a flame will respond to sounds, especially those of high pitch, produced in its neighbourhood, by sinking down to the wire-gauze. A flame which is highly sensitive and responsive to sounds too acute to be perceived by the human ear can be made by igniting gas issuing under great pressure from an exceedingly narrow jet.