RESEARCH METHODS IX STUDY OE FOREST ENVIRONMENT. Ill 
plete vapor saturation were attained, the absorption by the soils 
might be unlimited. 
"With partial vapor pressures over acid solutions, practical equi- 
librium was reached in all soils at the same ultimate moisture con- 
tent, whether the soil was started in a moist or chy condition. Thus 
Sea Island cotton soil, which was the finest used, dried out at a vapor 
pressure of 17.90 millimeters (76 per cent of saturation) in 97 days 
from 55 per cent to 6.08 per cent moisture, and the same soil ab- 
sorbed in the same period 5.6 per cent, starting from a dry condition. 
In the presence of a vessel of water the drying out was always very 
slow, and the fact that any drying whatever occurred is believed to 
be sufficient evidence that the atmospheres in the desiccator were not 
saturated, owing to the presence of the dry soils in the same atmos- 
pheres. 
Finally, the energy effects of absorption must not be overlooked. 
Patten and Gallegher cite a number of investigations which show 
that the heat released when vapor is absorbed by a soil is in excess 
of the latent heat which is released when vapor condenses. This fact 
indicates that water held in the soil, like water held in a solution, 
is brought to a greater density than that in which only water mole- 
cules are attracting each other. This density can only be obtained 
through the release of additional energy. 
Examined kinetically, then, the whole situation is fairly simple. 
Molecules of a gas or vapor repel one another, and this repulsion 
increases with the temperature and energy of each molecule. When 
a certain density is obtained in a volume of vapor, the so-called 
saturation density, the molecules may either return to the liquid 
from which they emanated or be compelled to unite with other 
molecules, starting condensation in the molecular sense. In the case 
of atmospheric vapor, solid particles, such as dust particles, may 
start condensation through their attraction for vapor molecules, 
which latter would otherwise repel one another too strongly to be 
brought together. 
The same phenomena occur in the soil. A soil particle of given 
size, mass, and gravitational power can attract to itself a certain 
number of vapor molecules, this number depending upon the space 
available and the distance at which the vapor-molecules begin to repel 
one another, or the temperature and energy of these molecules. A 
vapor molecule which has been trapped, and has given up some of 
its energy in this process of " individual condensation," is relatively 
inert, but not so inert as the soil particle, and is still capable of re- 
pelling other molecules to some extent. The ultimate number of 
molecules that can be held in a given soil, therefore, must depend 
(1) primarily on the energy of the free molecules as governed by 
temperature; (2) on the area or surface of soil particles exposed, 
