5 2 THE FIRST BOOK OF FARMING 



the difference in the two weights will be approxi- 

 mately the weight of the film water that the pebbles 

 can carry. Repeat this with sand and compare the 

 two amounts of water. A striking illustration can 

 be made by taking two slender bottles and placing 

 in them amounts of colored water equal to the 

 amounts of film water held by the pebbles and sand 

 respectively. In the accompanying illustration (Fig. 

 29), A represents the amount of water that was 

 found necessary to cover the pebbles in tumbler B 

 with a film of moisture. C is the amount that was 

 necessary to cover with a film the particles of sand 

 in D. The finer soil has the greater area for film 

 moisture. It has been estimated that the particles 

 of a cubic foot of clay loam have a possible aggre- 

 gate film surface of three-fourths of an acre. 



