RELATIONS OF THE SOIL TO HEAT, 193 
In Explanation of these observations we must recall to 
mind the fact that all bodies are capable of absorbing and 
radiating as well as reflecting heat. These properties, al- 
though never dissociated from color, are not necessarily 
dependent upon it. They chiefly depend upon the char- 
acter of the surface of bodies. Smooth, polished surfaces 
absorb and radiate heat least readily; they reflect 1t most 
perfectly. Radiation and absorption are opposed to each 
other, and the power of any body to radiate, is precisely 
equal to its faculty of absorbing heat. 
It must be understood, however, that bodies may differ 
in their power of absorbing or radiating heat of different 
degrees of intensity. amp-black absorbs and radiates 
heat of all intensities in the same degree. White-lead 
absorbs heat of low intensity (such as radiates from a ves- 
sel filled with boiling water) as fully as lamp-black, but 
of the intense heat of a lamp it absorbs only about one- 
half as much. Snow seems to resemble white-lead in this 
respect. Ifa black cloth or black paper be spread on the 
surface of snow, upon which the sun is shining, it will 
melt much faster under the cloth than elsewhere, and this, 
too, if the cloth be not in contact with, but suspended 
above, the snow. In our latitude every one has had op- 
portunity to observe that snow thaws most rapidly when 
covered by or lying on black earth. The people of Cham- 
ouni, in the Swiss Alps, strew the surface of their fields 
with black-slate powder to hasten the melting of the snow. 
The reason is that snow absorbs heat of low intensity 
with greatest facility. The heat of the sun is converted 
from a high to a low intensity by being absorbed and then 
radiated by the black material. But it is not color that 
determines this difference of absorptive power, for indigo 
and Prussian blue, though of nearly the same color, have 
very different absorptive powers. So far, however, as our 
observations extend, it appears that, usually, dark-colored 
soils absorb heat most rapidly, and that the sun’s rays 
9 
