CAPILLARY ACTION 93 



withdrawn from water comes out with a wet surface. Water 

 rises in a glass tube, or through the spaces existing in a mass 

 of sand, simply because the attraction of the surface particles 

 of the glass or sand for the particles of the water is at first 

 greater than the attraction of gravity ; the rise of water 

 ceases when the mass of water in the column reaches such 

 dimensions that the attraction of gravity balances the surface 

 attraction of the glass or sand. The surface attraction is 

 greater, and the quantity of water raised larger, the wider 

 the tube ; but the height to which the water is raised is 

 greater the narrower the tube ; because while the attracting 

 surface simply diminishes in the same ratio as the diameter 

 of the tube, the volume of water within the tube (and thus 

 the weight to be raised) diminishes as the square of the 

 diameter, and the less weight is thus carried to a greater 

 height. 



The same forces which occasion the rise of water in a tube 

 will determine the distribution of water over moist surfaces, 

 and this aspect of the subject is of considerable importance 

 for the correct understanding of the movements of water 

 in a soil. It is only when the interspaces of a soil are filled 

 or nearly filled with water, that an uninterrupted passage of 

 water through tubes of varying size, shape, and direction 

 can take place. In the case of a fully drained soil of open 

 texture, or consisting of coarse particles, the particles are 

 merely covered with a water film, and it is only at the points 

 of contact between the particles that anything of the nature 

 of a tube is to be found. The distribution of the water over 

 the surfaces of the moist particles is however governed by 

 the same laws which control its behaviour in soil tubes. 



Briggs (Mechanics of Soil Moisture, Soils, Bull. 10) has 



