56 



CAPILLAEITY 



In a concave surface the pressure exerted by the surface will be 

 less than that exerted by a horizontal or plane surface, because each 

 particle of liquid in the concave surface has not only the same forces 

 acting upon it as one in a plane surface, but, in addition, the attrac- 

 tive forces exerted by the particles contained in the liquid surround- 

 ing it, which is above the horizontal plane passing through the 

 particle shown in section by the black area in Fig. 2. The resultant 

 of all these attractions will be a small upward force. 



Thus while a particle, A, on a plane surface has a resultant force, 

 which we may call P, acting inward upon it, a particle, B, on a concave 

 surface has a resultant force, P, of the same magnitude acting upon it 

 inwards, but also another small force, which we may call p, acting 

 outwards. The final resultant force, therefore, acting upon a particle 

 on a concave surface is P p. 



As the whole free plane surface of the liquid outside the tube has 



FIG. 2. To explain true capillarity. 



thus a. force, P, acting upon each particle, it is evident that the pres- 

 sure per unit area of a plane surface is greater than that of a concave 

 surface. 



1 Consequently, the water is forced up the tube until the hydrostatic 

 pressure of the column, due to gravitation, balances p. 



I II This is true capillarity, i.e., the phenomenon shown by hair-like 

 tubes, and it is found that the smaller the diameter of the tube, the 

 higher will the liquid rise within it, because then the greater is the 

 concavity and, consequently, the greater is p. 



By similar reasoning it can be shown that the pressure exerted by 

 a convex surface is greater than that of a plane one. 



In a soil, the movements of water, though due to similar causes 

 (viz., surface pressure) as those which give rise to capillarity, are 

 mainly effected in quite a different manner. 



In many textbooks, it is said that the rise of water in a soil is 

 effected by capillary tubes existing in the soil. This is, in the writer's 



