mount of heat that accompanies fermentation, or
decay of vegetable matter, is seen in the case of rotting farmyard
manure. The danger of loss of the volatile ammonia from this cause is
often great, and care must be taken to prevent fermentation going on too
quickly, and the temperature from becoming too high.[44] The actual
increase in the temperature of a soil effected by the addition of
certain bulky organic manures, such as farmyard manure, may thus be
considerable. In some experiments carried out at Tokio, Japan, it was
found that the application of 20 tons of farmyard manure per acre
increased the temperature of the soil to a depth of five inches, for a
period of nearly a month, on an average, one and a half degrees
Fahrenheit. The amount of water present in a soil, it may be noticed in
passing, will have a considerable effect in regulating its temperature,
a damp soil being, as a rule, a cold soil.

_The Cause of the Heat of Fermentation._

It may be asked, How is the decay, or fermentation, of vegetable matter,
such as farmyard manure, caused? or rather, To what is it due? Decay of
any substance is just its slow combustion or burning. When a substance
unites with the active chemical element in air--the oxygen gas--it is
said to be oxidised. Now, this union of a substance with oxygen is the
explanation of burning, and the phenomena of burning and decay are
explained by the same chemical operation. When bodies decay, or when
they burn, they unite with oxygen: when this union of a body and oxygen
takes place very quickly, and the result is a flame and very great heat,
then we call it burning; when, however, it takes place slowly, it is not
called burning, but simply oxidation or decay. The ultimate products are
the same, however, whether the body burns or decays; and the process of
decay is always accompanied by heat, as well as the process of
burning.[45] It is not, of course, only the vegetable or organic matter
in a soil that decays, but also the mineral matter. The oxidation,
however, of the mineral matter in the soil takes place so slowly, and
the amount of heat generated by this oxidation is so slight, that the
temperature of the soil can scarcely be said to be much affected by it.

_Influence of Colour of a Soil._

There is still another quality of a soil on which its temperature
depends, and that is its colour. This may seem at first sight to be
scarcely worth taking into account, and yet it has been