THE FORMS OF SOIL-WATER 



161 



ture of the original film, and that at B, which is very acute 

 and which naturally must exert a very great outward pull. 

 Under the stress of this pull developed by the surface tension 

 acting in this film of very great curvature, the water is drawn 

 into the space between the particles, where it becomes thicker 

 than the capillary film about the particles. The readjustment 

 continues until the forces developed by the two films become 

 equal. An equilibrium is now established. In the soil the 

 tendency towards adjustment is somewhat similar in so far 



Fig. 28. — A conventional diagram showing the coalescence and read- 

 justment of the outer capillary water film of two particles when 

 Drought in contact. At the left is shown the condition before the 

 adjustment with a sharp angle at B; on the right, the films are at 

 equilibrium with a thickening at B due to movement from A and A'. 



as the outer capillary water is concerned. Complete equilib- 

 rium is probably never reached, however, due to constantly 

 disturbing factors. 



90. The determination of the amount of capillary 

 water in the soil. — The capillary water in a sample of 

 field soil may be determined by making a moisture test in the 

 ordinary way for the total water contained, 1 after the gravi- 



1 A moisture determination on a sample of field soil is generally carried 

 out as follows: — 100 grams of the sample, after thorough mixing, is 

 weighed into a suitable weighing dish and air-dried. The sample is then 

 placed in an oven and heated at 100°C or 110°C for four or five hours. 

 It is then cooled in a disiccator and weighed. The loss in weight is 

 water. The moisture is calculated as percentage based on the dry mat- 

 ter of the soil. If the weight of the water lost was 20 grams, the 

 percentage of moisture would be (20 -j- 80) X 100 or 25 per cent based 

 on dry soil. 



