The Romance of Modern Chemistry

HOW FIRE IS MADE they rust; the only difference is that the latter change is spread over a much longer time. There are other cases in which combustion appears to take place without any obvious cause, and to which the term “spontaneous” is applied. Stacks of hay occasionally take fire of their own accord, and heaps of cotton waste or rags impregnated with oil have been frequently found to exhibit a similar behaviour. But however “ spontaneous ” the occurrence may seem to be, there is in each case a sufficient reason for the combus- tion. In the first case the hay has been stacked while still moist, and in these circumstances fermentation sets in. Now fermentation is a chemical change of the con- stituents of the hay, promoted by the presence of minute organisms, and this change, like most chemical reactions, is accompanied by the evolution of heat If the hay were lying out in the open, this heat would be dissi- pated at once, but in the inside of a stack it cannot escape so easily; it accumulates more and more as the fermentation process goes on and ultimately the tempera- ture rises so high that the hay takes fire. The explanation is different in the case of the oily rags or cotton waste. Many oils are readily oxidised by the oxygen of the air, and when such oils are spread over the extensive surface of rags or waste the oxidation takes place very rapidly. The rags and waste being bad conductors, the heat generated in the oxidation is rapidly accumulated, and finally leads to the so-called “ spontaneous combustion.” In both these cases the chemical change involved is a slow combustion at the beginning, and becomes rapid at the end only because the heat generated in the pro- cess has been unable to escape. With rising tempera- 128