EXPERIMENT STATION BULLETINS. 505 



noted that the percentage of thermal motion of water increases more 

 than proportionally with temperature. The temperature of 40° 0. for 

 instance, is only twice as great as 20° O. while the percentage of mois- 

 ture moved is three times greater in the former case than in the latter. 

 Tlie water content of the various soils at which the maximum thermal 

 translocation occurs is 7.50 per cent for light sandy loam, 9.08 for heavy 

 sandy loam, 14.21 for silt loam, 18.80 for Clyde silt loam, and 19.29 for 

 clay. 



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 quantitj- of water is able to move from warm to 

 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 mois- 

 ture content. A thermal critical moisture content may be defined then 

 as that percentage of moisture in a soil which allows the greatest amount 

 of water to move from a warm to a cold soil at any amplitude of tem- 

 perature. 



A further examination of the preceding experimental data shows that 

 tlie thermal movement of moisture is extremely sensitive to the amount 

 of water present in a soil. It will be noted that by increasing or decreasing 

 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 soil in which the sensitiveness appears to 

 be more marked and the range more limited. If the increase in percent- 

 age of moisture content took place in this soil by 0.1% instead of 1.0% 

 the maximum thermal translocation would probably have been as high 

 as that of the other soils. It is possible, however, that the value ob- 

 tained 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 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 move- 

 ment would be greater at the highest moisture contents because there 

 would also occur a gravitational movement. .When soils containing as 

 high as 35.0 and 30.0 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° C. 

 and the other at 0° C. for 8 hours, such expectation as the above is not 

 at all unnatural. Instead, the water movement at these highest mois- 

 ture 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 mnnner, that the soils possess a very great attraction 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 mois- 

 ture in the soil is held by a force of great magnitude. 



