CAPILLARY ACTION 95 



surrounded by an elastic film of water exerting a considerable 

 pressure towards the centre of the particle. The result of 

 this state of tension is that an outward pressure exists on the 

 surface of the thicker portion of the film lying at the points 

 of contact between the particles. This outward pressure is 

 greater the thinner is the film on the adjoining particles. 

 If therefore moister and drier particles are in contact, water 

 will pass from the first to the last till an equilibrium of 

 surface tension is established throughout the system. The 

 figure shows this transference of water in action ; the arrows 

 indicate the direction of the pressure in the different parts 

 of the system. 



Water thus tends to distribute itself in a soil, either 

 through capillary passages, or by the slower process of 

 surface distribution. When these operations are assisted 

 by gravitation, as when rain falls on a dry soil, the move- 

 ment of the water becomes rapid. When these operations 

 are opposed by gravitation, as when a soil dries at the 

 surface and is still wet below, the movement is retarded, 

 and the amount of possible work is limited, as in the case 

 of the rise of water in a glass tube, by the final neutralization 

 of the attractive forces by the increasing weight of the column 

 of water lifted. 



The height to which water will rise in a soil by capillary 

 action depends primarily on the size of the soil particles, 

 and the closeness of their packing. When a soil is composed 

 of very coarse particles, as in the case of gravel, the inter- 

 spaces are so wide and the points of contact so few, that 

 capillary action has little influence on the distribution of 

 water throughout the mass. In the case of sands and loams 

 the interspaces become sufficiently narrow for the action to 



