Spontaneous combustion is an expression used to explain the setting on fire of a substance without the employment of any external agent, such as a lighted match, a flame, or a spark. To illustrate: There are times when a pile of coal will burst into flame without the application of a flame or spark. The reason for this is quite different from the reason for the burning of coal in a stove or under a boiler. The first burning is caused by spontaneous combustion, and the second by the ordinary combustion of coal. In both cases, however, the fires follow definite laws.

All combustion is a chemical action attended with the liberation of heat and is the result of the combination of oxygen with the combustible material. Ordinary combustion or burning is merely the result of a substance being heated, in the presence of a supporter of combustion like air, to the point of ignition by some external agent, such as a match. When a substance oxidizes with great rapidity, a great deal of heat is evolved and a flame is formed. The temperature at which the flame forms is known as the point of ignition or the kindling point.

Certain kinds of damp organic matter, such as soft coal or cotton rags containing oil, confined tightly may absorb enough oxygen to raise their temperature to the kindling point. The result is spontaneous combustion. The quantity of heat is the same whether the combustion is slow or fast. A quantity of wood that decays gives off exactly the same quantity of heat as if the same amount of wood were burned in a furnace, provided in both cases the wood is completely destroyed. The products of combustion are exactly the same.