52 ELEMENTARY AGRICULTURAL CHEMISTRY 



water is due to the same cause as that which produces capillarity, 

 but acting in a different way. If two particles of soil, each 

 covered with a thin film of closely adherent water, be caused to 

 touch, the water films at the point of contact must take up a 

 concave surface. At this point the pressure exerted by the 

 liquid surface is less than elsewhere. Consequently water 

 moves from the films around the particles and tends to accu- 

 mulate at the space between them until the curvature becomes 

 less, indicated by the dotted line (Fig. 3). The water is thus 

 held between the particles by a surface tension effect of the 

 same kind as that which causes capillarity. Now suppose three 



particles to be in contact, 

 each with its film of moisture, 

 and assume that the amount 

 of water held between B and 

 C (Fig. 4) is greater than 



that between A and B. The 



^j(j ^ water surfaces at the line 



of contact between A and B 

 will be more concave than those between B and C, and conse- 

 quei^ly the surface pressure exerted there will be less. Water 

 will therefore move round B in the direction of A until the 

 pressure exerted by the concave surface between A and B is 

 equal to that exerted between B and G, This action will occur 

 at every surface between neighbouring particles, and will account 

 for the movements of water from particle to particle, even 

 though there be many interstices filled wdth air. 



It is to be noted that the movements of water due to this 

 cause may be in any direction ; it will practically always be 

 from a more wet to a less wet part of the soil. The structure 

 of soil, therefore, in respect to the cause of the rise of water 

 from below is to be regarded in this way, and not by assuming 

 that the sol' particles form hair-like tubes filled with water. 

 The quantity of water which is held by a soil and the readiness 

 with which the water is raised from below is thus largely 

 dependent upon the number of points of contact of the soil 



