144 
ABSORBING AND EMITTING SURFACE. 
a. Absorbing and Emitting Surface. 
A given area of ground, as estimated by the every-day rule 
of measurement in yards or acres, presents always the same 
apparent quantity of absorbing, radiating, and reflecting sur¬ 
face ; but the real extent of that surface is very variable, 
depending, as it does, upon its configuration, and the bulk and 
form of the adventitious objects it bears upon it; and, besides, 
the true superficies remaining the same, its power of absorp¬ 
tion, radiation, reflection, and conduction of heat will be much 
affected by its consistence, its greater or less humidity, and its 
color, as well as by its inclination of plane and exposure.* 
* Composition, texture and color of soil are important elements to be 
considered in estimating the effects of the removal of the forest upon its 
thermoscopic action. “Experience has proved,” says Becquerel, “that 
when the soil is bared, it becomes more or less heated [by the rays of the 
sun] according to the nature and the color of the particles which compose 
it, and according to its humidity, and that, in the refrigeration resulting 
from radiation, we must take into the account the conducting power of 
those particles also. Other things being equal, silicious and calcareous 
sands, compared in equal volumes with different argillaceous earths, with 
calcareous powder or dust, with humus, with arable and with garden earth, 
are the soils which least conduct heat. It is for this reason that sandy 
ground, in summer, maintains a high temperature even during the night. 
We may hence conclude that when a sandy soil is stripped of wood, the 
local temperature will be raised. After the sands follow successively ar¬ 
gillaceous, arable, and garden ground, then humus, which occupies the 
lowest rank. If we represent the power of calcareous sand to retain 
heat by 100, we have, according to Schubler, 
For [silicious ?] sand. 95.6 
“ arable calcareous soil. 74.3 
“ argillaceous earth. 68.4 
“ garden earth. 64.8 
“ humus. 49 .O 
“ The retentive power of humus, then, is but half as great as that of 
calcareous sand. We will add that the power of retaining heat is propor¬ 
tional to the density. It has also a relation to the magnitude of the par¬ 
ticles. It is for this reason that ground covered with silicious pebbles 
cools more slowly than silicious sand, and that pebbly soils are best suited 
to the cultivation of the vine, because they advance the ripening of the 
grape more rapidly than chalky and clayey earths, which cool quickly. 
