50 EG ) T TL 1 y 1 1 G R 1C UL TUNE. 



persists there. Why does not all the water fall off ? The 

 reason is that there are forces acting in the surface of the 



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film which balance the weight of the water. 



(2) If we touch the above particle with a second, we 

 find the film spreads over the two continuously. 



(3) If the two particles are unequal in size the water 

 stands more deeply on the small one. 



Now applying this knowledge to the case of soils we see 

 that it is by the surfaces of the particles that a soil may 

 hold water. 



Now the area of the surface of the particles will be 

 greater in a fine soil, and we expect too the depth of the 

 film to be greater, and so a fine soil ought to hold more 

 moisture than a coarse one. This we know to be the case 

 in practice. A clay soil holds more water than a gravel. 



The creeping or spreading of the water film from one 

 particle to another takes place in all directions until a 

 condition of equilibrium is reached. Now if the thickness 

 of a film is interfered with at any place there is at once a 

 flow of liquid to restore the balance. Thus if we remove 

 a little water from one particle the others next it at once 

 share some of theirs with it. 



This affords an explanation of the rise of water in the 

 soil. The particles above touch the film below and the 

 water in it is shared with them, so the film spreads upwards 

 and draws continuously on lower supplies. 



The question arises Why then does not the water rise 

 quite to the surface in all cases ? This is met by the fact 

 that the resultant of the forces in the surface of the film 

 is finite and so can only support a given weight. It has 



