8 JOHN JOHNSTON AND L. H. ADAMS 



apparent to all who have adduced capillary effects as a means of 

 accounting for geological phenomena. 



(A) Since the pressure discontinuity occurs only at the surface- 

 of separation, a column of liquid (as in Fig. 2, for example) can be 

 supported only when there is a free liquid surface within the capillary; 

 in the case of porous materials, therefore, only when there are 

 surfaces of separation within the pores. ^ 



(B) When equihbrium has been established, the height attained 

 by the liquid in a tube depends only upon the bore of the tube at 

 the surface of separation (since it is this which determines the 

 curvature) and not in any way whatever on the size or shape of 

 the rest of the tube. This statement however by no means impHes 

 that liquid will rise in material containing pores of irregular diameter 

 to the height corresponding to the width of pore observed at the 

 surface of a fragment of the material. 



(C) Consider an open tube, shorter than the height of column 

 of water which it would support, which is filled with water and 

 placed so that its lower end dips below a mercury surface. The 

 free water surface assumes a curvature sufficient only to support 

 the existing column of water; but if water be removed from the 

 top (by evaporation, or otherwise) the curvature becomes greater, 

 and consequently mercury is pulled up into the tube. This process 

 continues until the surface is hemispherical, when it supports a 

 column of mercury and water equivalent in weight to the (much 

 longer) column of water alone which it would support. This is 

 virtually the atmometer, the only difference being that in the latter 

 there are a very large number of pores; we see moreover that the 

 height at equilibrium is determined by the width of the largest 

 pores in the material at the surface of separation. 



ip) The pressure discontinuity at the surface of separation — • 

 which we may look upon as a pressure exerted by the surface film 

 of water in an endeavor to contract itself — ^is precisely the same in 

 amount whether it make itself manifest {a) by supporting a column 

 of liquid; {h) by compressing the air in a tube closed at one end 

 and wholly immersed in water; (c) in the form of the pressure 



' This of course includes the case where the liquid in the pores extends practically 

 to the surface of the material as a whole. 



