Spontaneous combustion is the burning of a substance or body by the internal development of heat without the application of fire.

It not infrequently takes place among heaps of rags, wool and cotton when sodden with oil; hay and straw when damp or moistened with water; and coal in the bunkers of vessels.

In the first case, the oil rapidly combines with the oxygen of the air, this being accompanied by great heat. In the second case, the heat is produced by a kind of fermentation; and in the third, by the pyrites of the coal rapidly absorbing and combining with the oxygen of the air.

The term is also applied to the extraordinary phenomenon of the human body, which has been told of some people, whereby it is reduced to ashes without the application of fire. It is said to have occurred in the aged and persons that were fat and hard drinkers, but most chemists reject the theory and altogether discredit it.