RETENTION OF SOIL MOISTURE 



147 



tendency is to arrange upon the surface of the soil 

 particles a film of water molecules equivalent to this 

 thickness. 



But, because of the second set of forces the film is 

 always thicker than this range of molecular attraction 

 of the solid. This is due to the attrac- 

 tion of the water particles for each 

 other, or cohesion. The water mole- 

 cules hang together. This cohesion of 

 the water molecules is exhibited in 

 surface tension which will permit a 

 clean steel needle to be suspended 

 upon the surface of water, or makes 

 possible the common trick of putting 

 a handful of nails into a goblet already 

 level full of water. This surface ten- 

 sion acts like a stretched elastic mem- 

 brane, and permits the water to be 

 piled up. This is what happens in 

 the soil when capillarity comes into distribution of water 



1 J on columns or spnen- 



play. As a result of these two sets ca ' particles of differ- 



r J ent texture. Note the 



of attraction, the water hangs on the accumulation of water 



in the lower part, also, 



particles in thick films; and it drops the approximately 



r r equal curvature of water 



away only when the weight of the surfaces at each level, 

 water becomes greater than the surface tension of the 

 liquid. 



It is clear that soil forms a column of considerable 

 height, and further, that the closer the water film is 

 drawn around the soil particles, the thinner it will be, 

 and consequently the less water it will contain. To 

 illustrate: Suppose a cylinder to have flexible rubber 



Fig. 45. Showing the 



