COMBUSTION is chemically the rapid oxidation of a substance with the evolution of light and heat. It is generally accompanied by flame and the production of carbon dioxide, carbon monoxide and the vapor of water. The substances which are consumed are popularly termed combustibles. Chemically, the sub stances oxidized are the carbon and hydrogen components of the combustibles. The heat in combustion is a result of the chemical changes which take place during oxidation. The light is emitted by incandescent particles of carbon in the flame. The temperature to which any particular substance must be raised in order that it shall take fire is called the °ignition point° of that substance. Ignition points vary with the different combustibles. They may even be below the temperatures which ordinarily pre vail. In such case the substance will burst into flame when that temperature is reached, and this phenomenon is called "spontaneous com bustion.° Some kinds of vegetable and animal oils when spread out thinly, as on a rag, may oxidize so rapidly as to become very hot, and if in a confined place will eventually take fire spontaneously.
In combustion two classes of combustibles are recognized: (1) Those which in the proc ess generate heat sufficient to continue the combustion until the whole of the combustible is consumed; (2) those in which the combus tion can only be kept continuous by the con stant application of external heat. When the processes of combustion take place instantane ously the combustion is called an explosion. Acetylene may be mentioned as an example in this connection. It has a tendency, especially when under pressure, to decompose, with the evolution of so great a degree of heat as to expand enormously and result in an explosion. The economic use made of combustion is chiefly for the generation of power, but also in cooking and the heating of houses and the production of all artificial lights but electric light. The combustibles utilized in this way are called fuels. See EXPLOSION; FUELS; Fua NACE ; GAS; HEAT; LIGHT; OXYGEN; PHLOGIS TON. Consult Hayes, J. W., 'Combustion and Smokeless Furnaces' (Chicago 1915).