IV. THE REACTIONS OCCURRING IN SOILS 55 



cally no influence on the herbage beyond 1 ft. over the boundary of 

 the plot. 



2. The liquid in the soil moves from two chief reasons : 



(a) Motion due to Surface Pressure. The motion of water in a 

 soil, due to surface pressure, may be understood from the following 

 considerations : The particles of a liquid exert an attraction upon each 

 other, but this attraction is exerted only through a very short distance. 

 If a particle, A (Fig. 1) of water, well below the surface of a mass of 

 water, be considered, it will be 



seen that the attractions of Q D 



other particles will be exerted f jo */ 



equally in all directions, and it '-.. i- ..-' 



is possible to conceive of the 

 particle being surrounded by a i< >: 



sphere beyond which the at- \. I ,-*' 



traction of other particles be- 

 comes negligible. But in the FlG ' l To ex P lain surfftce P^ure. 

 case of a particle, B, on the actual surface, CD, of the liquid, it will be 

 seen that there remains a resultant attraction, in a direction at right 

 angles to the surface, pulling the particle inwards towards the mass of 

 the liquid. Thus the surface of a liquid exerts considerable inward 

 pressure (estimated in the case of water at ordinary temperatures to be 

 about 1300 atmospheres). Any mass of liquid therefore tends to take 

 up a form which has the least surface, i.e., a sphere. But under 

 ordinary conditions this tendency is overpowered by other forces, e.g., 

 gravitation. 



A liquid surface always assumes a form at right angles to the re- 

 sultant of all the forces acting upon it. Since gravitation is usually 

 the largest force, large masses of liquid have a truly horizontal surface. 

 When a liquid touches a solid body, one of two things happens, either 

 the liquid wets the solid or it does not. In the first case, e.g., with 

 glass and water, the attraction of the solid for the liquid particles is 

 greater than that of the liquid particles for each other ; in the second, 

 that of the liquid particles for each other than that of the solid for the 

 liquid particles. 



In the former case, the surface of the liquid near the solid is de- 

 formed and becomes concave, owing to the previously horizontal sur- 

 face being pulled up the surface of the solid. 



In the latter case, the liquid surface becomes convex near the solid, 

 owing to the superior attractive force exerted by the liquid upon the 

 particles near the solid. 



In both cases, the surface remains always at right angles to the 

 resultant of all the forces acting upon it, gravitation having added to 

 it, near the solid, other forces, in one case directed laterally toward the 

 solid, in the other, away from the solid. 



If a tube of glass be" immersed in a liquid, both the interior and ex- 

 terior of the walls of the tube will be wetted and the surface of the 

 liquid will therefore be deformed. If the tube be a narrow one, the 

 liquid within the tube will have a concave surface, as shown in Fig. 2. 



