CHAPTER XV. 



WATER OF SOILS 



I. HYGROSCOPIC MOISTURE 



WATER is held in soils by the manifestation of attractive force 

 under three forms: (1) hygroscopic, or adhesion; (2) surface 

 tension, and (3) hydrostatic, or gravity. These give rise to the 

 three so-called forms of water hygroscopic, capillary, or film, and 

 gravitational. ' It must be remembered that the water of all is the 

 same in chemical composition, the only difference being in the force 

 holding or moving it in the soil. 



Hygroscopic Moisture. All substances have the power of con- 

 densing moisture upon their surfaces, hence a very thin film of 

 water exists around all substances exposed to the air. This phenom- 

 enon is known as adsorption. The water is held very firmly by ad- 

 hesion or molecular force, which is estimated as equal to 10,000 

 atmospheres, and the water may be removed only by a temperature 

 much higher than the ordinary. When normal temperature is 

 restored, the moisture will again be slowly condensed upon the sur- 

 face. Briggs 1 has calculated the thickness of the hygroscopic film 

 for quartz particles as 2.66 X 10~ 6 centimeters, or 0.0000266 milli- 

 meter. The amount of hygroscopic moisture in a soil depends upon 

 several factors. 



Hygroscopic Capacity of Soils 2 



(a) Size of Particles. The amount of hygroscopic moisture 

 in soils varies inversely as the size of the particles and directly as 

 the internal surface of the soil. Since colloids are made up of very 

 minute particles, a small amount present will increase very mate- 

 rially the internal siirface and consequently the total amount of 

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