172 NATURE AND PROPERTIES OF SOILS 



continues to be withdrawn at A, this adjustment goes on with 

 considerable ease until the film channel (cc'c") becomes so 

 thin as to cause its surface now (bb'b") to approach very 

 closely to the surface of the soil particle and the inner capil- 

 lary water. The sluggishness of the water movement becomes 

 a factor at this point, impeding the capillary adjustment to- 

 ward A. This point of sluggish capillary movement has been 

 designated by Widtsoe 1 as the point of lento-capillarity, and 



Fig. 32. — Conventional diagram for the explanation of the effect of the 

 thickness of water film about the soil particles and their colloidal 

 complexes on the ease of capillary adjustment. 



is expressed in percentage based on the dry weight of the 

 soil. It lies near the transition zone between the inner and 

 outer capillary water. 



The amount of capillary water delivered at any one point, 

 therefore, will obviously be influenced by the thickness of the 

 film and may consequently be taken as a measure of rate of 

 adjustment. A short soil column should deliver more water 

 than a longer one, due to the thicker films at the surface of 

 the former. King, 2 in studying the evaporation from the sur- 

 faces of sand columns of different lengths, their bases being 

 in contact with free water, obtained some significant data. 



1 Widtsoe, J. A., and McLaughlin, W. W., The Movement of Water in 

 Irrigated Soils; Utah Agr. Exp. Sta., Bui. 115, pp. 223-231, 1912. 



2 King, F. H., Principles and Conditions of the Movements of Ground 

 Water; U. S. Geol. Survey, 19th Ann. Kept., Part II, p. 92, 1897- 

 1898. 



Also Briggs, L. J., and Lapham, M. H., Capillary Studies; U. S. 

 Dept. Agr. Bur. Soils, Bui. 19, pp. 24-25, 1902. 



