RELATIONS OF THE SOIL TO HEAT. 189 
the surface of the earth radiates into the cooler atmos- 
phere and planetary space; the temperature of the surface 
rapidly diminishes, and the air itself becomes cooler by 
convection.* As the cooling goes on, the vapor suspend- 
ed in the atmosphere begins to condense upon cool objects, 
while its latent heat becoming free hinders the too sudden 
reduction of temperature. The condensed water collects 
in drops—it is dew; or in the colder seasons it crystallizes 
as hoar-frost. 
The deposition of liquid water takes place not on the 
surface of the soil merely, but within it, and to that depth 
in which the temperature falls during the night, viz., 12 
to 18 inches. (Krutzsch observed the temperature of a 
garden soil at the depth of one foot, to rise 3° F. ona 
May day, from 9 A. M. to 7 P. M.) 
Since the air contained in the interstices of the soil is at 
a little depth saturated with aqueous vapor, it results that 
the slightest reduction of temperature must at once occa- 
sion a deposition of water, so that the soil is thus supplied 
with moisture independently of its hygroscopic power. 
Conditions that Affect the Temperature of the Soil.— 
The special nature of the soil is closely connected with 
the maintenance of a uniform temperature, with the pre- 
vention of too great heat by day and cold by night, and 
with the watering of vegetation by means of dew. It is, 
however, in many cases only for a little space after seed- 
time that the soil is greatly concerned in these processes. 
So soon as it becomes covered with vegetation, the char- 
* Though liquids and gases are almost perfect non-conductors of heat. yet it can 
(Mffuse through them rapidly, if advantage be taken of the fact that by heating they 
expand and therefore become specifically lighter. If heat be applied to the upper 
surface of liquids or gases, they remain for a long time nearly unaffected ; if 
it be applied beneath them, the lower layers of particles become heated and rise. 
their place is supplied by others, and so currents upward and downward are 
established, whereby the heat is rapidly and uniformly distributed. This process 
of convection can rarely have any influence zn the soil. What we have stated 
concerning it shows, however, in what way the atmosphere may constantly act 
in removing heat from the surface of the soil, 
