IMPORTANCE OF CAPILLARY MOVEMENT 173 



is correspondingly cut down. This factor of friction 

 greatly limits the effective capillary capacity of a soil 

 both vertically and horizontally. If the coefficient of 

 friction is great, it will soon overcome the pull due to 

 curvature, and water will be quickly moved in from 

 only a short distance. In proportion as the friction 

 coefficient is reduced, the range of movement is ex- 

 tended. It should be noted that friction retards move- 

 ment rather than stops it. The greater the surface 

 over which a given volume of water is spread, the slower 

 therefore will be its movement. (See page 183.) 



In the above discussion it was assumed that the 

 water is uniform in all its properties, and therefore 

 that corresponding curvatures were the same. If, how- 

 ever, anything modifies the surface tension of the liquid 

 at one point as change of temperature, solution, etc., 

 this would be expected to disturb the balance, and result 

 in film movement. Such is the case, as later examples 

 will show. (See page 183.) It is probable that, in the 

 soil, equilibrium is never established, because of these 

 disturbing variations all through the soil mass. Further, 

 the last end of the process of adjustment is exceedingly 

 slow, and probably never actually takes place; because 

 the force producing the motion is successively reduced 

 as equilibrium is attained, and because the difference in 

 curvature of the films is so slight. 



83. Extent, rate and importance of capillary move- 

 ment. Capillary movement of water is of great conse- 

 quence to growing plants. Since it concerns the capillary 

 water, it affects that form of soil water upon which 

 ordinary crops are directly dependent. The withdrawal 



