150 Journal of Agricultural Research voi.v,no.4 



and cohesive forces, due entirely to temperature, between the warm 

 and cold columns of soil are equal in amount, as in the soil with the 

 lowest moisture content. Water, therefore, tends to move from the 

 warm to the cold soil. Inasmuch as the attraction of the soil has been 

 further satisfied and the water films further thickened, the pull of the 

 cold soil, due only to the attractive forces of the soil for water, is decreased; 

 on the other hand, the ease with which the warm soil gives up moisture 

 is increased. The result is that even though the total effective pull 

 (composed of the increased surface tension of water, the increased attrac- 

 tive adhesive forces of soil for water, and the force of the cun^ature of the 

 capillary films) of the cold soil mth the high moisture content is less than 

 that of the soil with low moisttjre content, the greater ease with which 

 the warm soil with high water content parts with moisture enables the 

 reduced effective pull to draw more water from the warm to the cold 

 side. As the moisture content of the soil is continually increased, its 

 attractive power is satisfied and the curvature of the capillary films 

 decreased correspondingly. The total effective pull of the cold column 

 of soil is continually decreased, but the ease with which the warm column 

 of soil gives up moisture is also continually increased, so that the thermal 

 translocation of water is constantly increased with rise in moisture 

 content. 



Finally, a degree of moisture content is reached in which the effective 

 pull of the cold column of soil is able to extract the greatest amount of 

 water from the warm column of soil. This degree of water content is 

 the thermal critical moisture content. At this point the attractive 

 power of the soil for water is considerably satisfied but is far from being 

 entirely appeased; the total effective pull of the cold column of soil is 

 also considerably less than that of the preceding columns of soil, but 

 the warm column yields water to this pull wnth such ease that there 

 occurs a maximum thermal water translocation. Inasmuch as the 

 water-attractive power is different for the various kinds of soils, 

 this thermal critical moisture content is of necessity also different. 

 After this thermal critical moisture content is reached, the effective 

 pull of the cold column of soil is further decreased with a continued 

 increase of moisture content. And although the willingness of the 

 warm column of soil to part more readily with moisture is also 

 increased, yot the pull of the cold column of soil is not sufficiently 

 strong to draw it; consequently the thermal movement of water 

 commences to decrease and continues to diminish very regularly and 

 gradually with a continued increase in moisture content. When the 

 highest percentage of water is reached, the warm soil is very willing to 

 part with a very large amount of water, but since the effective pull of the 

 solid soil is reduced almost to a minimum, only a small amount of mois- 

 ture is drawn from the former to the latter. 



The degree of moisture of the different soils could not be further in- 

 creased, on account of the difficulty of sifting them, and consequently it 



