CHAPTER XVI 



WATER OF SOILS 



II. CAPILLARY WATER 



THE most abundant and by far the most important form of soil 

 moisture is capillary or film moisture. It differs from hygroscopic 

 moisture in that it evaporates at ordinary temperatures, is not con- 

 densed again on the soil particles, and may move from one particle 

 to another. 



The term capillary as applied to this form of water has arisen 

 from the fact that this movement may be best seen in very 'small 

 capillary tubes. When tubes are placed in water the height to which 

 it will rise varies with the diameter of the tube. 



Height of Capillary Rise in Glass Tubes 



Diameter of tubes 



1.0 nun. 

 .1 mm. 



Height of water 



15.336 mm. 

 153.36 mm. 



.01 mm. 1533.6 mm. 



The law expressing this action is as follows: The height to 

 which the water rises varies inversely as the diameter of the tube. 

 The reduction of the diameter one-tenth causes the water to rise 

 ten times as high. The movement of water in soils from a free 

 water surface resembles somewhat the movement of water in a large 

 number of capillary tubes of various sixes. In the case of soils 

 where the water rises from a free water surface the amount in the 

 soil varies inversely as the distance above the free water. This 

 would be exactly true of a large number of various-sixed capillary 

 tubes. The explanation for soils is not quite as simple, however, 

 as this would indicate. 



Surface Tension. Whenever an air-water surface exists the 

 molecules of water in the interior are attracted equally in all direc- 

 tions. The molecules on the surface are subjected to a double but 

 unequal attraction of the water on one side and the air on the other. 

 Avbich has the effect of producing a thin film composed of the sur- 

 face molecules which is under tension. Tf the film is flat no pres- 



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