THE FORMS OF SOIL WATER 211 



temperatures, being held with less tenacity ; and (3) in 

 that it has the power of movement from place to place 

 within the film, hence the name capillary water. 



140. Surface tension and the force developed thereby. 

 - The power that tends to hold this capillary water in 

 place against the force of gravity, a constant, depends 

 on the surface tension of the liquid. This phenomenon 

 of surface tension is due to the existence of certain molec- 

 ular forces acting from within. In a drop of water, for 

 example, the particles are attracted equally in all direc- 

 tions and consequently are able to move with perfect 

 freedom. The molecules on the surface of the drop, 

 however, are not in such an equilibrium of attraction, 

 since the pull of the water particles within is greater than 

 that of the air particles without. The resultant attrac- 

 tion is therefore inward, and is directed along a line per- 

 pendicular to the surface at that point. The result is 

 the development of a more or less ideal membrane, the 

 effective force of which is not affected by the amount of 

 the surface, but by the curvature. In a sphere the force 

 or pressure developed by surface tension is equal to twice 

 the surface tension divided by the radius. This increase 

 of the effective force by curvature of film is very impor- 

 tant as regards soil water, since, as will be shown later, it 

 governs the movement of capillary water from one particle 

 to another, the direction of the movement being deter- 

 mined by a difference in pressure as developed by un- 

 equal curvatures of film surfaces. 



As a result of this force developed by surface tension, 

 the water film around a soil particle tends to equalize 

 itself until this pressure is everywhere the same. On 

 this force depends also the thickness of the capillary film. 

 Under any given condition this capillary film will con- 



