THE FORMS OF SOIL WATER 227 



near approach to the conditions of a field soil. Since this 

 is rather a difficult study to carry out, most of the rate 

 and height data on capillary movement have been largely 

 obtained with dry columns in contact with free water at 

 the bottom. Such data are comparative, but are far 

 from quantitative as regards the performance of any soil 

 under normal conditions. 



153. Surface tension and capillary movement. — As 

 has already been shown, the thickness of a maximum 

 capillary film is largely determined by surface tension; 

 and as surface tension with any given curvature exerts a 

 definite pressure, it is evident that this pressure may be- 

 come greater or smaller with variations in the surface 

 tension. One of the most potent factors having to do 

 with this variation is temperature. If the temperature 

 of a soil. column in capillary equilibrium and containing 

 its maximum capillary moisture should be raised, some 

 of the water would be lost as free water, since the pulling 

 power of the films would be decreased. In the same 

 way, the capillary capacity would be increased by a lower- 

 ing of the temperature, which of course would mean a 

 higher capillary rise in either a dry or a wet soil. The 

 rate of movement, 1 however, would be facilitated in the 

 first case, since the viscosity of the water would be much 

 reduced, allowing the movement in the film channels to 

 take place with less friction. 



King 2 has verified these conclusions in his experiments 



1 Wollny, E. Untersuchungen iiber die Kapillare Leitung 

 des Wassers im Boden. Forsch. a. d. Gebiete d. Agri.-Physik, 

 Band 8, Seite 206-220. 1885. 



2 King, F. H. Fluctuations of the Level and Rate of Flow 

 of Ground Water. U. S. D. A., Weather Bur., Bui. 5, pp. 59- 

 61. 1892. 



