RELATIONS OF THE SOIL TO HEAT. 195 
ence between the absorption of radiant heat of low inten- 
sity or its communication by conduction on one hand, and 
that of high intensity like the heat of the sun on the other. 
Retention of Heat.—Other circumstances being equal, 
the power of retaining heat (slowness of cooling) is the 
greater, the greater the weight of a given bulk of soil, 
i. e., the larger and denscr its particles. 
A soil covered with gravel cools much more slowly 
than a sandy surface, and the heat which it collects during a 
sunny day it carries farther into the night; hence gravelly 
soils are adapted for such crops as are liable to fail of rip- 
ening in cool situations, especially grapes, as has been 
abundantly observed in practice. 
Color is without influence on the loss of heat from the 
soil by radiation, because the heat is of low intensity. 
The porosity or roughness of the surface (extent of sur- 
face) determines cooling from this cause. Dew, which is 
deposited as the result of cooling by radiation of heat into 
the sky, forms abundantly on grass and growing vege- 
tation, and on vegetable mould, but is more rarely met with 
on coarse sand or gravel. 
Influence of Moisture on the Temperature of the Soil. 
— All soils, when thoroughly wet, seem to be nearly alike 
in their power of absorbing and retaining warmth. This 
is due to the fact that the capacity of water for heat is 
much greater than that of the soil. We have seen that 
lime sand and quartz sand are the slowest of all the in- 
gredients of soils to suffer changes of temperature when 
exposed to a given source of heat. (See table, p. 194.) 
Now, water is nine times slower than quartz in being 
affected by changes of temperature, and as the entire sur- 
face of the wet soil is water, which is, besides, a nearly 
perfect non-conductor of heat, we can understand that ex- 
ternal warmth must affect it slowly. 
Again, the immense consumption of heat in the forma- 
tion of vapor (see note, p. 188) must prevent the wet soil 
