So THE TEXTURE OF THE SOIL [chap. 



ment then takes place till the stretched film regains the 

 same tension everywhere, which is effected by a motion 

 of the oil to the place where the tension has been 

 increased. If the withdrawal of the oil be continued, the 

 film round the balls becomes thinner and thinner ; the 

 more it is stretched, the more closely it clings to the 

 surface, so that the removal becomes progressively more 

 difficult ; at last the film becomes so much stretched 

 that it ruptures and reunites again over a smaller 

 surface, hence with a diminished tension. The rupture 

 naturally takes place where the film is thinnest, on the 

 top layer of balls, which becomes more or less " dry " 

 while the lower balls are still surrounded by their 

 film. 



Just in a similar way water will always move in a 

 soil from a wet to a dryer place, till the film surround- 

 ing the particles is equally stretched throughout. 



For example, if A, B, C (Fig. 6) represent three soil 

 particles, of which A and B are surrounded by a thin, 



FlG. 6. Diagram illustrating Liquid Film round Soil Particles. 



and C by a thicker, film of water : when the spheres 

 are in contact the water will fill up part of the angle 

 between the spheres, as shown in the diagram. But 

 the water surface at a is more curved than at b, i.e., 

 it corresponds to the surface at a in the fine capillary 



