168 NATURE AND PROPERTIES OF SOILS 



92. Capillary movement of water. — It has already been 

 shown that different thicknesses of capillary films tend to 

 equalize in the soil due to the pulling forces developed by the 

 angle of curvature between the particles. 1 It is evident that 

 differences in curvatures must be the motive force in the capil- 

 lary movement of soil-water. Let it be supposed, for conveni- 

 ence, that three equal spheres when brought in contact contain 

 unequal amounts of water in the angles of curvature (see 

 Fig. 31). In this case the greater pull would exist at A, since 

 the angle here is more acute. Consequently water must move 



per cent., while 1 per cent, of silt corresponded to a retention of only 

 .13 per cent, of water. Eepresentative data is as follows: 



Briggs, L. J., and McLane, J. W., The Moisture Equivalent of Soils; 

 U. S. Dept. Agr. Bur. Soils, Bui. 45, 1907. 



1 An ingenious method for measuring quantitatively the capillary pull 

 exerted by a moist soil has been devised by Lynde and Dupre\ The 

 apparatus consists of a glass funnel joined to a thick- walled capillary 

 tube by means of a piece of rubber tubing, a water seal being used at 

 this point. The lower end dips into mercury. The soil to be studied is 

 placed in the funnel, and after being saturated is connected by means 

 of a wick of cheesecloth or filter paper to the water column previously 

 established in the capillary tube. If no break occurs between the soil 

 and the capillary water column, the apparatus is ready for use. 



The excess water having drained away, there is a thinning of the films 

 on the soil surface due to evaporation. Equilibrium adjustments now 

 take place, which result in the drawing upward of the water column. 

 The mercury follows, and the strength of the pull may be measured by 

 the length of the mercury column. The old method of measuring capil- 

 lary power by the water movement through a dry soil is vitiated by two 

 conditions — the length of time necessary, and the fact that the maximum 

 lift cannot be obtained due to excessive friction. This new method 

 uses a wet soil, requires only a short time, and gives a more nearly 

 accurate idea of the power of the capillary pull. It does not, however, 



