150 A TREATISE ON METAMORPHISM. 



Molecular attraction. — The fourth force affecting the movement of ground 

 water is molecular attraction. This attractive force works between the 

 particles of water themselves (cohesion) and between the particles of water 

 and rock (adhesion). 



As a result of molecular attraction water may rise against gravity in 

 capillary or hair-like openings, thus saturating the rocks at higher altitudes 

 than it would were it not for this cause; it may creep along the walls of 

 the openings of rocks without extending from wall to wall, and therefore 

 without saturating the rocks. 



The rise of water when it fills capillary openings raises the free surface 

 of water above the normal level. This rise of the free surface is explained 

 by the attraction between the water and the walls, and the attraction of the 

 molecules of water for one another. The strong attraction between the 

 surfaces of mineral grains and water has already been alluded to. As a 

 result of this, water tends to rise along a wall or tube. This is dependent 

 upon the fact that there is greater attraction between the molecules of rock 

 and water (adhesion) than between the molecules of water themselves 

 (cohesion). However, the molecular attraction between the particles of 

 water is very great. The strength of the surface tension of a film of pure 

 water is dependent upon cohesion, and is 81.96173 dynes per square centi- 

 meter." When a molecule is surrounded on all sides by free water the 

 attractions in the various directions equalize one another, and so particles 

 are comparatively free to move However, at the surface the upward com- 

 ponent of the attraction is zero; hence there is effective tangential and 

 downward attraction. The rise of the water along the walls is due to 

 adhesion. As a result of this attraction a film of water is drawn along the 

 walls. Because of the attraction of cohesion the film of adherent water 

 draws up the next row of molecules away from the walls; these molecules 

 in turn exert an attractive force on the next adjacent molecules, and so on. 

 The attractive force of the surface film of water for the water below draws 

 up the molecules constituting it; this in turn acts upon the film below, and 

 so on. The total effect of the molecular attraction between the walls and 



aPaniell, Alfred, A text-book of the principles of physics, 3d ed., Maenrillan Co., New York, 

 1895, pp. 271-279. Oatwald, W., Outlines of general chemistry, translated by James Walker, 3d ed., 

 Macmillan Co., New York, 1895, pp. 107-111. Barker, Geo. F.', Physics, Holt & Co., New York, 1892, 

 pp. 200-211. 



