Because the hay, having become damp, decays, and passes on to a state of fermentation, in which chemical changes occur, during which heat is evolved. Hay, taking fire under these circumstances, would exhibit spontaneous combustion.
"Who hath woe? who hath sorrow? who hath contentions? who hath babbling? who hath words without cause? who hath redness of the eyes? * * * They that tarry long at the wine."—Prov. xxiii.
317. What substances are liable to produce spontaneous combustion?
All substances which contain sugar, starch, and other components liable to fermentation. All bodies that evolve, under low degrees of temperature, inflammable gases. And all organic bodies undergoing decay.
Grain, cotton, hemp, flax, coals, oily and greasy substances.
318. What is the Ignis Fatuus (sometimes called "Will-o'-the-Wisp", "Corpse Candles," and "Jack-o'-Lantern")?
It is a flame produced by spontaneous combustion, caused by the decay of animal or vegetable bodies, which evolve phosphoretted hydrogen gas, under circumstances attended by a low degree of heat, sufficient to ignite the gases. It is mostly seen over marshy places, and burial-grounds.
Many a "Ghost Story" has owed its origin to these singular but harmless appearances. People, ignorant of the cause, have been terrified at the effect. To the fancy of an affrighted mortal, the simple flame of the Ignis Fatuus has assumed the form of a departed friend, and even found a supernatural voice. If, excited by a momentary daring, the beholder moved towards the light upon which he gazed, it fled from him. If he turned from it and walked away, it followed him, step by step. The darkness of a lonely road, or the sacred solitude of a burial-place, have been sufficient accessories to authenticate the appearance of a spirit. And yet how simple the phenomenon? Matters so volatile as those which produce the Ignis Fatuus would naturally be driven back by the motion in the air caused by an advancing body; and, on the other hand, a body moving from them would create a current in which the Ignis Fatuus would follow. Poisonous gases, escaping from decaying bodies, pass into the air and take fire. They are thereby converted into harmless compounds. Thus we see that the "ghost" which terrifies the mind of the ignorant, becomes a "guardian angel" to the educated.