26 
It is certain that soil moisture distributes itself around the soil 
particles and in some instances as an impregnation within the par- 
ticles. The moisture therefore occurs as thin films which, according 
to certain physical conceptions, must be held around the particle by 
an enormous pressure. From purely physical considerations this 
pressure has been estimated at several thousand atmospheres. Under 
such pressure the concentration of film water with reference to the 
mineral matter should be much greater than that of the free or 
capillary water in the soil. 
Then the air-dried soil, the particles of which are still surrounded 
by a film of moisture, when shaken with water, should theoretically 
show the least solubility. The results reported in this bulletin in 
most instances are in harmony with this assumption. But if the soil 
be allowed to remain in the condition and environment prevailing in 
submerged cultures, that is, in the presence of a large excess of water, 
then in time diffusion would bring about a more or less equal distribu- 
tion of dissolved materials throughout the entire water present and 
the pressure of soil films would be decreased to a minimum or entirely 
eliminated. Hence the amount of materials going into solution in 
the free water in such soils would be expected to be abnormally high. 
Upon air drying such soils the normal films would again appear with 
a resulting decrease in solubility. Subsequent heating ought then 
to affect these soils in a way similar to that produced on dry-land 
soils. The data presented in the previous tables are again in harmony 
with this view. 
Water, however, not only exercises a solvent action on minerals 
but forms various hydrates, the solubility and physical character of 
which in some instances are greatly altered ; organic as well as inor- 
ganic matter goes into solution with the result that the moisture films 
around the particles became solution films, holding in suspension and 
more or less intermingled with colloids, both organic and inorganic. 
The films then may be looked upon as being of a colloidal nature. 1 
Upon heating to 100° C. alterations in the films would take place 
through evaporation and by partial dehydration of colloids, thus de- 
stroying the pressure by which the film was previously held around 
the particles. At the temperature of 100° C. the concentration of the 
soil moisture would also be temporarily increased, due to increase in 
solubility with heat. During the course of the evaporation the con- 
centration of the soil moisture would increase to the saturation point, 
after which the mineral matter would be deposited on the surface of 
the film as evaporation went on. 2 Also the materials held in solution 
in the interior of the permeable particles would be partially deposited 
i No claim is made for originality in this view. The idea of soil films, colloidal films, etc., has been made 
use of by various writers on soils. 
* King (loc. cit.) in discussing the relative solubilities of fresh and dried soils advanced this idea. 
