94 



RELATIONS OF SOIL TO WATER 



given a very clear account of the condition and behaviour 

 of water in a drained soil. The retention of water on the 

 surface of soil particles, in spite of the opposing force of 

 gravitation, is due to what is called * surface tension.' The 

 surface of the film of water encircling a soil particle is 

 really in the condition of an elastic membrane exerting a very 

 considerable pressure ; the water is in consequence firmly held 



FIGURE 3. 



against the soil particle. In a fully drained soil there exists 

 a condition of equilibrium between the force exerted by 

 surface tension and the force exerted by gravity. If the 

 films of water became thicker and heavier, a part of the 

 water would gradually pass downwards in obedience to the 

 attraction of gravity. If the films became thinner they would 

 acquire the power of absorbing and retaining fresh supplies 

 of neighbouring water. 



In Figure 3 will be found the diagram used by Briggs 

 to illustrate the transference of water from a wetter to a drier 

 portion of the soil, a transference which in this illustration 

 depends entirely on the surface tension of the water, and 

 not upon the existence of connecting tubes. 



The three particles of soil, which lie in contact, are each 



