in.] PERCOLATION OF WATER 79 



continuous film of wetted surfaces to lead the water 

 down by surface tension. The top layer of soil becomes 

 thoroughly wetted and will not allow the air below to 

 escape ; only after some time are local displacements of 

 the air set up, which enable the water above to make 

 connection with the wetted subsoil below, so that 

 percolation can begin. For this reason summer rains 

 falling in a season of drought are often noticed to be 

 of little benefit to the crop, because they are retained 

 near the surface until wholly evaporated, instead of 

 increasing the stock of moisture at the lower levels 

 where the roots of the plant are then operative in 

 obtaining water. 



Minimum Capacity of Soil for Water. 



The amount of water retained in a soil by surface 

 tension alone, when percolation has removed as much 

 as possible, is rather an important factor to determine, 

 as upon it depends to some extent the power of the 

 soil to resist drought by retaining water for the crop 

 between intervals of rain. If short columns of sand and 

 soil which have been saturated and then allowed to 

 drain away into a state of equilibrium are considered, 

 it is clear that very different proportions of water 

 are retained by the various soils. Suppose, however, 

 the column be of such a length that at some level the 

 upper film of water cannot be further stretched, but 

 the particles cease to be wet; the layer immediately 

 below this dry soil contains the minimum amount of 

 water consistent with a continuous film at all. Soil 

 in this condition may be regarded as at the minimum 

 of saturation; it will part with no more water by 

 drainage, and will become drier only by evaporation. 

 In order to obtain such a sample of soil for determina- 



