64 SOILS AND MANUKES 



amount, the reverse action takes place ; the force of 

 surface tension overcomes that of gravity, water is drawn 

 upwards, and the films become thicker until equilibrium 

 is established. This phenomenon is called capillarity, 

 because it resembles the rise of water in fine tubes (Fig. 8) 

 and is due to the same cause. 



There is, of course, no free water surface underneath 

 the soil, but excess percolates downwards and evaporation 

 takes place from the surface. The lower layers of s-oil, 

 therefore, generally contain more water than those above, 

 and some of it is drawn upwards by capillary action. But 

 if the upper layers contain more water than the lower as 

 when rain falls on very dry soil the force of surface 

 tension acts in the same direction as gravity and the water 

 descends very rapidly under their combined influence. 

 Equilibrium is constantly disturbed by the introduction 

 and abstraction of water, and the water is constantly 

 moving downwards or upwards according to the conditions. 

 The surface tension of the water causes it to move from 

 any point where there is more to where there is less. The 

 movement takes place laterally as well as vertically, and 

 tends to equalise the distribution of the water in the soil. 



The apparatus employed for the study of capillary 

 phenomena in the laboratory consists simply of a series of 

 glass tubes each about 60 inches long by 1 inch in 

 diameter, and closed by a piece of linen tied over one end. 



The tubes are filled with dry soil and fixed in vertical 

 position with the closed ends immersed in water (Fig. 9). 



From observations made in this way it has been con- 

 cluded that capillary action depends mainly upon the size 

 and arrangement of the particles. In general, the smaller 

 the particles and the more numerous the points of contact, 

 the greater the height to which the water will rise and the 

 greater the quantity of water which will rise to a given 

 height. The rise takes place, at first, more slowly in fine 



