Oct. 25, 1915 Temperature and Capillary Moisture in Soils 147 



Obviously, then, the maximum thermal v/ater 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 themial 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 o.i instead of 

 i.o 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 Vv^ater 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 0° 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 bv a force of great magnitude. 



