74 THE TEXTURE OF THE SOIL [chap. 



layers is largely due to the pull from the oil below, 

 while in the lowest layer of all the whole tension 

 exerted by the stretched film is devoted to holding up 

 its own thick film of oil. If oil be taken away at any 

 point, the curvature of the film, and therefore the tension 

 of the surface in that region, is increased : a readjust- 

 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. 5) represent three soil 

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

 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 6, i.e., 

 it corresponds to the surface at a in the fine capillary 

 tube (Fig. 3) as compared with the surface at b in the 

 wider tube. But the diminution of pressure caused by 

 a is greater than that caused by fr, as shown by the 

 greater height to which water is raised in the tube ; 

 hence in the same way the pressure inside the liquid 



