Hay, when piled damp and in too large masses, ferments and turns dark. In decomposing, sufficient heat is developed to be insupportable when the hand is thrust into the mass, and vapors begin to be emitted. When the water is almost entirely evaporated, the decomposition continues, and the hay becomes carbonized little by little; and then the charred portion, like peat-peat-cinders mixed with charcoal, sulphurous pyrites and lignite, etc.

Becomes a kind of pyrophorus, by virtue of its great porosity and of the large quantity of matter exposed to high oxidation. Under the influence of air in large amount, this charcoal becomes concentrated on the surface to such a degree that the mass reaches a temperature which results in its bursting into flames. The preventives for this danger are care that the hay in the lofts is kept perfectly dry, that it is well stacked, and that it is stored in small heaps rather than in large masses.