WATKl: 



89 



is called the film moisture. This does not drain off 

 and serves to keep the soil moist, supplying water for 

 growing plants. The amount of water held as film 

 moisture varies greatly in different soils. 



Pour water over a pile of broken stones or coarse 

 gravel. Each stone soon becomes surrounded by a film 

 of water; then the pile will hold no more water, and if 

 more is added it simply runs off. Exposed to the air 

 or sun, the pile of stones soon dries out, and the 

 moisture disappears, leaving the stones as dry as before 

 they were wet Now, pour water on a lump or two of 

 dry i -l;iy, of about the same weight as the stones, and 

 notice how much more water it absorbs than the stones. 

 After the clay becomes thoroughly wet, it may be 

 exposed to the air and sun a long time before it dries 

 out. The clay consists of a greaei number of small 

 particles, each of which, when wet, becomes covered 

 with water; nu.»>t of the air spaces between the particles 

 are also filled, and in this way nracfa iratei ii taken up. 

 As only the out it } articles are exposed to the air and 

 sun the tamp dries out slowly. The pile of stones, on 

 the other hand, consists of a small number of coarse 

 particles, and consequently holds less film moisture* 

 The water quickly drains from the air spaces. Leering 

 each particle exposed to the air, which quickly evapo- 

 rates tli" film moisture, 



B Cornell Experiment Station has published two 



interesting drawings illustrating the amount of water 



held by different soils. This experiment was made by 



the Station Chemist, and is described as follows: 



" He put small marbles in a tumbler, as shown by 



