Q. Why do hay-stacks sometimes catch fire of themselves?
A. Either because the hay was got up damp, or because rain has penetrated the stack.
Q. Why will a hay-stack catch fire if the hay be damp?
A. Damp hay soon decays, and undergoes a state of fermentation; during which, carbonic acid gas is given off, and the stack catches fire.
Q. Why does roasted coffee sometimes catch fire spontaneously?
A. The heat of coffee is greatly increased by being roasted; and the carbon of the coffee uniting with the oxygen of the air, produces carbonic acid gas, and bursts into flame.
Q. Why do old rags, used for cleaning lamps and candles, sometimes set a house on FIRE?
A. Because they very readily ferment, and (during fermentation) throw off exceedingly inflammable gases.
(N.B. Lamp-black mixed with linseed oil is more liable to spontaneous combustion, than anything that servants handle.)