CAPILLARY MOVEMENT 169 



subsoil will deflect its movement. Entrapped air will 

 do the same thing, and this has been found to be a 

 potent source of contamination of open wells with shal- 

 low curbing. This is particularly true in heavy soils, 

 where the escape of entrapped air is especially difficult. 

 One of the beneficial effects of under drains is that they 

 facilitate the entrance and movement of rain-water 

 in the soil by affording a channel for the escape of 

 entrapped air. (See page 241.) 



81. Capillary, or film movement. Capillary water 

 has been described (see page 141) as occurring in the 

 soil in a thin film overspreading the particles, and thick- 

 ened into a waist-like form at their points of contact. 

 Toward the bottom of any soil column the film is always 

 thicker than at the top, owing to the less weight which 

 the surface tension must bear. This form of distribution 

 has given rise to the term film water, from which is 

 derived the idea of film movement, to describe this 

 type of capillary movement. 



Film movement expresses very accurately the actual 

 condition of affairs, for if there is any translocation of 

 water at this stage it must be through this film. 



82. Principles governing capillary movement. It 

 will be remembered (page 147) that, when equilibrium 

 is established in any mass of wet soil short of saturation, 

 the water surfaces are comparable to a stretched elastic 

 membrane. The more closely this film is drawn about 

 the particles, the more surface there is exposed, and the 

 greater pull the surface tension exerts. Consequently 

 the greater the amount of water which will be retained. 



In a soil capillarily saturated with water there is 



