212 SOILS: PROPERTIES AND MANAGEMENT 



tinue to thicken until the mass of the water is so great 

 as to allow gravity to come into play and pull enough 

 water away to again restore the equilibrium. The soil 

 particle would at this point he maintaining its maximum 

 thickness of capillary film. It is also quite evident that 

 as the capillary Him is thinned — as, for example, by 

 evaporation — the force developed by surface tension 

 would be increased, due to increased curvature of the 

 film, and the difficulty of removing the external layers 

 of the film would naturally become greater. 



141. The form of water surfaces between soil particles. 

 — In the case of a soil, however, the question of the 

 capillary film becomes more complex, since a great num- 

 ber of different-sized particles are present in more or less 

 close contact with one another. This means that under 

 normal soil conditions the capillary film is continuous 

 from one particle to another — a very different question 

 to consider from that of a film about a single isolated soil 



grain more or less 

 spherical in shape. 

 Suppose, for example, 

 that two particles, 

 each earning a capil- 

 lary water film, be 

 brought into such 

 contact that the films 

 coalesce. There are 

 now two distinct sur- 

 faces — that at A, A' (see Fig. 32), with the curvature 

 of the original film, and that at B, which is very acute 

 and which naturally must exert a very great outw 7 ard 

 pull. Under the stress of this pull developed by the 

 surface tension acting in this film of very great curvature, 



Fit;. 32. Diagram showing the coalescence 

 aw) readjustment to the capillary film of 

 two soil particles when brought in con- 

 tact. At left is shown the condition be- 

 fore adjustment with a sharp angle at B ; 

 at right the films are shown in equi- 

 librium with a great thickening at B. 



