50 ELEMENTARY AGRICULTURAL CHEMISTRY 



at the surface. The imaginary limit of the particles which 

 have any appreciable effect on D would be represented by a 

 similar sphere, or, rather, by a hemisphere, since above the 

 Burface there are no liquid particles. The resultant of the 



B 





Fig. 1. 



forces acting upon Z) will be a considerable force acting towards 

 the body of the liquid and at right angles to the surface. The 

 same would apply to all particles on the surface, and to a less 

 degree upon all particles nearer to the surface than the radius 



'■-V 



Fig. 2. 



of the imaginary hemisphere. Hence a plane surface exerts a 

 pressure upon the liquid within it. 



Now take a surface which is concave, and consider a 

 particle, A (Fig. 2), on the surface. Draw a circle, with 

 A as centre, to represent the imaginary limit beyond which 

 the attraction of particles for A becomes inappreciable. 



