What is spontaneous combustion?

  Spontaneous combustion is the ignition of substances apparently without any direct cause. The spontaneous combustion of organic materials is sometimes a cause of fires. When large quantities of such materials as soot, linen, paper, cotton or woolen stuffs, or ship's cables become soaked with relatively small amounts of oils (especially drying oils) and exposed to a limited access of air, they may take fire sooner or later. The presence of moisture frequently aids spontaneous combustion, and piles of damp hay, freshly mown grass, sometimes take fire spontaneously through the fermenting action of certain bacteria.