Fire is heat and light produced by the combustion of Inflammable substances. Combustion is a chemical operation carried on in the air, or the chemical union of the oxygen of the air with some combustible body, like hydrogen gas or the solid carbon, and is attended with the evolution of heat and light The heat and the light come from the sun. With every particle of vegetable matter that is formed by the combined action of the sun and the carbonic acid gas in the air, a portion of the sun's heat and light is absorbed and held fast in it. And whenever this vegetable matter is decomposed, - as in burning wood, coal, or oil, which are only definite forms of vegetable matter, - this heat and light are given out. The amount of each depends upon the mode of burning.
Air is composed mainly of two elementary gases, oxygen and nitrogen (one part oxygen and four parts nitrogen), with a small amount of watery vapor and carbonic acid gas.
Pure oxygen is a gas which has a wonderful attraction for, and power of combination with, every other element. If it were everywhere present in a perfectly pure state, it would consume or burn up everything; but it is diluted or mixed (not combined) with nitrogen, another gas which is incombustible, and which lessens the combustibility of everything with which it comes in contact. Owing to this dilution, the oxygen will not unite with the carbon and hydrogen with which it is everywhere surrounded, and produce rapid combustion, except at a high temperature. The temperature at which this union takes place is called the burning-point, and this varies in different substances. Thus combustion is within the power and control of man; and some extra means are usually employed to increase the temperature to the burning-point, - friction, or percussion, or the use of some more highly inflammable substances, like sulphur and phosphorus. This produces heat sufficient to complete the chemical union, or, in common phrase, "kindles the fire."
The heat generated for all household purposes is produced by the chemical action of the oxygen of the air upon the hydrogen and carbon which are found in the various kinds of wood and coal. The oxygen first combines with the carbon and decomposes it, producing carbonic acid gas, which escapes into the air, from which it is absorbed by plants, or by human lungs when there is no proper ventilation. The oxygen also combines with the hydrogen gas in the fuel, and this produces the flame; the larger the amount of hydrogen in the fuel, the greater the amount of flame. Some of the products of combustion are not entirety consumed, and pass off as smoke; some are incombustible, and remain as ashes. The intensity of a fire and the amount of heat which it produces are always in proportion to the amount of oxygen with which it is supplied. There should be just air enough for perfect combustion. An excess of air projected upon a fire conveys away the heat, cools the fuel, and checks the combustion. The supply of air should be controlled by confining it in a limited space.
Fires are usually kindled at the bottom of a flue or chim-ney. The heated air, being lighter, rises; the colder, denser air rushes in to take its place, becomes heated, and ascends. Thus a continuous current is established, and a constant supply of fresh air secured. The chimney serves to carry off the smoke and poisonous products of combustion; the heavier, incombustible products settle in the form of ashes. The force of this current of air drawing through the chimney (a matter of great importance) is called the draught. It varies with the temperature and amount of air in the room, and the length and width of the chimney.