398 Capillary Rise of Water in Soils 



cross section is an equilateral triangle of side K, assuming that the tube 

 is wetted by the water so that the angle of contact is zero. 



A simple calculation^, ignoring small corrections, gives the value as : 



where T — surface tension, 



p = density of the water, 



g = force of gravity, 

 and h, K have the meanings assigned above. 

 The cross sectional area of the tube is 



4 ' 



and no considerable error will be made by putting this equal to the mean 

 value •2118?-* deduced by Slichter for the soil-pore. 

 Thus 



^.A-^=-2118r^ (2), 



whence A' = -Ir very nearly. 



Substituting the value of A in (1): 



■Ipyr pyr 



It is known that the effect of most dissolved salts is to increase the 

 surface tension of water. In the present case, however, the soil is assumed 

 saturated and the soil solution therefore very dilute. We can thus employ 

 the usual value 75 dynes cm.^ with very little error. 



Taking p = 1 and g = 981 we obtain a value for h: 



^ '.= -:° w 



We can substitute various values for /•, the radius of the soil grain, in 

 this equation and obtain the corre.sponding capillary rise, h. It is 

 instructive to take for this purpose the various grain sizes as obtained 

 by the usual mechanical analysis. The results are given in Table II. 



It will be seen that the possible height of capillary rise rapidly in- 

 creases as the diameter of the grains diminishes. Equation ( i) from wliicli 



■i See for instance Poynting and Thomson, Properties nf Mniter, 4th ed. 10O7. p. 141, 

 where the calculation is given for a circular tube. Exactly the same principle holds for a 

 triangular tube. 



