THE MOISTURE IN THK SOIL 59 



illustrated by filling straight (or argand) lamp chimneys with 

 compacted dry soil and standing them in a dish of water. 



68a. Film moisture can be illustrated by dipping a marble 

 into water and observing the skin or film of moisture adhering to 

 all sides. The most satisfactory conditions of soil moisture exist 

 when each soil grain is covered by a film of water. The char- 

 acter of film moisture is changed by the thickness of the film. 

 The thicker the film, the less the tension to the body, until 

 it becomes so thick as to separate from that body and become a 

 drop of water ; and it is then subject to the law of gravitation, 

 and can travel but in one direction downward. While in a state 

 of film moisture, it is amenable to the law of capillary attraction, 

 and can move in any direction, which means that it goes towards 

 the thinnest films. The readiness with which water films travel 

 can be seen by dipping a piece of cube sugar into coffee and 

 observing how quickly the liquid pervades the lump of sugar. 

 That soil moisture may move with the same facility as the 

 coffee does in the sugar, it is necessary to have the soil grains 

 in proper touch one with another ; not so far apart but that 

 the water films can reach one to the other, not so close as to 

 impede the progress of the films. The two extremes in soil can 

 be seen in loose gravel and hard clay. 



70a. By rainfall is meant precipitation, the fall of water in 

 any form, as in rain, snow and hail. 



72a. That different soils vary in their capacity to hold 

 moisture may be illustrated by the following experiment : Pro- 

 vide several flower-pots of the same size and shape. The va- 

 rious soils should be thoroughly dried in an oven. At least 

 four kinds of soil should be tested: gravel, sand, clay, and gar- 

 den loam. Place an equal weight of each soil in the pots. 

 Suspend one of the pots from a common spring-scales (Fig. 11). 

 Notice the number of pounds and ounces registered. Now 

 slowly pour water upon the soil until it is thoroughly saturated. 

 Cover with a piece of oiled cloth or oiled paper, and allow it 

 to drain until no more water will flow from it. The water 

 which drains from the pot is the free water. The difference in 

 weight of the pot of soil before soaking, and after the drainage, 

 shows the amount of water held by capillarity. 



