180 WATER AND FOREST. 



these have the same percentages of total volumes and areas In grains 

 of uniform size, the rate of the passage of water will be higher the 

 larger the grains. With very minute grains the passages become 

 capillaries entirely, gravitation is overcome. The larger the grains 

 the nearer the flow approaches that of free water acting under gravita- 

 tion only. 



Capillary action is one of surface tensions. Imagine a membrane 

 enclosing each grain and stretched thereon. The tension of this imag- 

 inary membrane is analogous to surface tension. It can be demon- 

 strated by very simple mathematical reasoning that the surface ten- 

 sions increase with decrease of radius — the sharper the curvature the 

 greater the tension. When neighboring interstitial spaces are filled 

 with water to a greater or less degree, surfaces of films of sharper or 

 flatter curvatures are produced. The surfaces are not in equilibrium, 

 ard a movement from the flat to the sharp curves takes place and con- 

 tinues until by readjustment of the curves equilibrium is established. 

 This is the nature of capillary action, it takes place in all directions 

 according to the surrounding conditions. Gravity always acts down- 

 wards. In soils the conditions are such that capillary action is usually 

 upwards. The limit of the action varies with the texture of the soil. 

 In chalk the limit is not reached at sixteen feet. In sandy soil one and 

 one-half feet has been found to be the extreme. In very open mate- 

 rial the limit may be but a few inches. 



In further illustration, imagine a tall box filled with soil com- 

 pletely saturated with water — and the bottom of the box be suddenly 

 removed. The water drains away with gradually decreasing speed. 

 At first a stream, and then drops decreasing in frequency. Capillary 

 action is opposing gravity. The topmost spaces are being emptied 

 by water leaving films around each grain. As the depletion continues 

 the films draw closer to the grains, their curvature grows sharper, 

 their tension increases. They exert an increasing upward pull on those 

 below, and these in their turn begin to pull on those below, and so on 

 down the full depth of the box. Presently the chain of films exert 

 together suflicient force to balance the attraction of gravitation. The 

 dropping from the box ceases. 



Add more water to the crop — the interstices are re-flUed, the films 

 destroyed or flattened in curvature, the surface tension is lessened. 

 The films below lose all or part of their support from those above. 

 The attraction of gravity prevails, the dropping re-commences, chang- 

 ing to a stream if the supply at the top be ample. The speed with 



