THE SOIL 49 



sulphates, and potassium salts than ammonium or calcium 

 compounds. 



2. The Motion of the Liquid itself, — The solution in a soil 

 moves from two causes : 



(i) Gravitation. — This is the attraction of the earth for 

 water, and only acts vertically downwards, tending to make the 

 solution sink into the soil. 



(ii) Surface Pressure. — Every liquid surface exerts a 

 pressure upon the liquid within it. This is readily understood 

 when it is remembered that in a liquid every particle is 

 attracted by its neighbours, with a considerable force by those 

 particles nearest to it, and with less and less force by particles 

 more and more distant. Those at the surface are only 

 affected by the attraction of those within the liquid ; the 

 surface, therefore, is under a pressure acting inwards. A 

 liquid free to move always arranges itself so as to have the 

 smallest possible surface. If no other forces were acting it 

 would always assume a spherical form. Under ordinary con- 

 ditions gravitation far overpowers the effects of surface pres- 

 sure, but if the quantity of liquid be small the gravitation effect 

 becomes small and surface pressure asserts itself ; the liquid 

 thus tends to become spherical. Solid substances often exert 

 an attractive force (adhesion) upon liquids, so that when they 

 touch a liquid they become wetted. 



The pressure exerted by a liquid surface depends upon its 

 form ; it is less than that of a plane if the surface be concave, 

 greater if convex. This can be understood by reference to the 

 diagram. 



Let A B (Fig. 1) represent a plane surface of a liquid, and 

 consider any particle, C, well below the surface. The particles 

 whose attractive forces for C have any appreciable magnitude 

 may be assumed to lie within an imaginary sphere encircling 

 C, and represented in section by the circle. It is evident that 

 the resultant of all the attractive forces acting upon G will be 

 nil^ since it will be attracted equally in all directions. It 

 therefore remains in equilibrium. Now consider a particle, Z>, 







