VI.] SURFACE OF SOIL PARTICLES 105 



particles, the bigger area there is to which water, or 

 anything else wetting the surface, will cling; further- 

 more, the smaller the particles into which a given 

 weight of soil is divided, the greater the total area of 

 surface. Imagine a little cube, i inch on the side, 

 divided by three cuts into eight cubes, each \ inch on the 

 side ; the surface exposed by the solid material has been 

 doubled, because for one cube with six faces, each an inch 

 square, there are now eight cubes, each with six faces a 

 quarter of a square inch in area, or 12 square inches in 

 all. In consequence, a really fine-grained soil possesses 

 an enormous area and a corresponding absorbing power ; 

 it has been calculated in various ways that the particles 

 contained in a cubic foot of an average heavy soil 

 possess surfaces making up a total area of about an 

 acre, while in a clay soil this is increased to about 4 

 acres. All surface-tension actions are, therefore, more 

 powerful in clay and similarly fine-grained soils; they 

 absorb a greater amount of water, and hold on to it 

 more tightly. The rate of motion of water, however, in 

 such soils is slower, whether it be the downward move- 

 ment due to gravity and surface tension together which 

 we call percolation, or the upward movement due to 

 surface tension alone when the surface is losing water. 

 The slowness of the movements is due to extreme 

 fineness of the channels between the minute clay 

 particles; the water in these channels cannot move 

 freely, because it is never removed from the drag caused 

 by the attracting surfaces of the solid. Skin friction is 

 always at work, and there is never anything approaching 

 free flow. 



Applying these principles to the soils themselves, we 

 see that in a coarse-grained sandy soil percolation will 

 always be rapid, because the mere motion of water by 

 gravity in the large spaces between the particles will be 



