MOLECULAR ATTRACTION AND GROUND CIRCULATION. 151 



the water and the molecular attraction between the particles of water is to 

 produce an elevation above the normal surface of the water, the upper 

 surface of which is of a shape as though it were an elastic membrane 

 adhering- to the walls and being stretched by the weight of the water above 

 the ordinary level below. 



The height to which water rises above this natural level is indirectly 

 as the diameter of the capillary openings. In circular glass tubes 1 mm. in 

 diameter, at 20° C, pure water rises 3.32 cm." Between plates 1 mm. apart 

 it rises half of this amount. Since the height is inversely as the diameters 

 of the openings, in circular tubes 0.01 mm. in diameter, the height in tubes 

 would be 3.32 m. and in sheet openings 1.66 m. 



In circular tubes 0.001 mm. in diameter the height in tubes would be 

 33.2 m., and in sheet openings 16.6 m. ; and in circular tubes 0.0002 mm. in 

 diameter — that is, openings of a size intermediate between subcapillary and 

 capillary — the water would rise to a height of 166 m, and in sheet open- 

 ings 83 m. Since many rocks have openings as small as or even smaller 

 than this, capillary attraction may be very important in the position of the 

 ground- water level. (See pp 411-412.) If the openings are inclined the 

 lengths of the openings thus filled are correspondingly great. 



The height to which the water rises is independent of the character of 

 the walls, provided the walls are wetted, 6 and hence the above numbers are 

 applicable to rocks. However, the height to which the water rises dimin- 

 ishes as the temperature increases; hence, the above numbers should be 

 modified somewhat as the top of the sea of ground water has a temperature 

 below or above a temperature of 20° C. Ordinarily this modification is of 

 minor importance. 



Above the level to which the water may be raised as a continuous sheet 

 in the capillary openings, the water may still creep along the walls of the 

 openings without filling them. The obstinacy with which a film of water 

 holds to the rock surface has already been explained. This water is that of 

 imbibition (p. 124). In proportion as the water of imbibition varies in 

 amount the water under molecular attraction creeps from areas of greater 

 humiditv to areas of less humidity. To the rise of the free surface due to 

 capillarity there is a definite limit; there is no limit to the creep of water 

 along the walls. It is presumable, however, that such movement is rela- 



a Barker, cit, pp. 209-210. b Barker, cit., p. 210. 



