Oct. 25, 1915 
Temperature and Capillary Moisture in Soils 
147 
Obviously, then, the maximum thermal water movement depends 
upon a definite condition of moisture of any particular soil; a deviation 
from this definite degree of moisture in either direction causes a decrease 
in thermal movement of water. Since this definite percentage of 
moisture at which the greatest quantity of water is able to move from 
a warm to a cold column of soil appears to be a specific constant or 
characteristic of the various soils, it is proposed to designate it as “thermal 
critical moisture content.” A thermal critical moisture content may be 
defined, then, as that percentage of moisture in soil which allows the 
greatest amount of water to move from warm to cold soil at any ampli¬ 
tude of temperature. 
A further examination of the preceding experimental data shows that 
the thermal movement of moisture is extremely sensitive to the amount 
of water present in a soil. It will be noted that by increasing or decreas¬ 
ing the percentage of soil water by small degrees, the thermal movement 
varies very markedly in either direction. From this it follows that the 
thermal critical moisture content must be quite definite, and in order to 
obtain it absolutely, the percentage of soil moisture near the point of 
maximum thermal movement must be increased by small amounts. 
This applies especially to the light sandy soils, in which the sensitiveness 
appears to be more marked and the range more limited. If the increase 
in percentage of moisture content took place in this soil by 0.1 instead of 
1.0 per cent, the maximum thermal translocation would probably have 
been as high as that of the other soils. It is possible, however, that the 
value obtained is about the upper limit for this soil and consequently for 
all soils of its type. 
The diminution of the thermal translocation of water with a decrease 
in moisture content from the point of thermal critical moisture content 
might be anticipated, but the decrease of water movement with further 
increase of moisture content after the point of thermal critical moisture 
content was not expected. Indeed, it was at first thought that the 
movement would be greater at the highest moisture content because 
there would also occur a gravitational movement. When soils contain 
as high as 35 and 30 per cent of moisture, as did the Clyde silt loam 
and the clay, respectively, and when one half of their column is kept 
at 40° and the other at o° C. for eight hours, such expectation as the 
above is not at all unnatural. Instead, the water movement at these 
highest moisture contents is very low and in descending order and the 
cessation of diminution is not as yet reached. These results go to show, 
then, in a most striking manner that the soils possess a very great attrac¬ 
tion for water and that their requirements for water to satisfy their 
attractive forces before free movement of water can take place are, 
indeed, high. Until the point is reached where gravitational movement 
occurs, the moisture in the soil is held by a force of great magnitude. 
