3. From various physical and chemical processes which take place in it
and create heat.
[Sidenote: Heat through sun’s rays.]
[Sidenote: Dark soils absorb more heat than light-coloured soils.]
[Sidenote: Capacity for heat is greater in damp and fine-grained
soils.]
[Sidenote: Evaporation and condensation of aqueous vapour produce the
greatest effect in fine-grained soils.]
Dealing with the upper layers of the crust, it may be said that,
besides the intensity of the sun’s rays, the temperature also depends
on a variety of properties possessed by various kinds of soil, amongst
which latter may be mentioned the absorption of heat, which is much
greater in dark than in light-coloured soils; the heat conductivity
and the capacity for heat, which lead to higher temperatures in damp
and fine-grained soils; and finally the evaporation and condensation
of aqueous vapour, which tend to prevent extremes of heat and cold and
which likewise produce the greatest effects in fine-grained soils.
[Sidenote: A fine-grained damp soil does not get so hot, but retains
the heat better.]
It follows from these observations that a coarse-grained, dark
coloured and dry soil will show the highest and lowest temperatures,
whereas a fine-grained damp soil does not get so hot but retains the
heat better.
[Sidenote: The temperature of the surface of the soil may exceed that
of the air.]
It ought to be pointed out in this place that a variety of
circumstances may bring about very high temperatures on the surface of
the ground which considerably exceed the average temperatures of the
air at the same time.
[Sidenote: Laws regulating the subsoil temperatures.]
Concerning the laws that have been deduced from careful and long
continued observations of subsoil temperatures, it will not be
necessary at this point to deal minutely with them; it must on the
contrary suffice to summarise only the more important ones.
With the distance from the surface of the ground,