194 Mr. J. T. Riley on Capillary Phenomena. 



We may suppose that, while the drop of ether is held over 

 the plane surface, the constitution of the liquid near the 

 surface is continually changing, from the passage of the ether 

 molecules into the upper layers. 



The term E7 is meant to include the whole of whatever 

 plane-surface actions may exist, tending to produce an internal 

 pressure. It is evident that K7 ought to be considerably less 

 than K, since H 7 is very much less than H under the same 

 conditions of surface.. 



The equation of the capillary surface is then practically 

 reduced to 



H/l 1\ 



where p is the hydrostatic pressure immediately below the 

 capillary surface. 



All that we know experimentally concerning the forces 

 which produce capillary phenomena may be expressed in two 

 fundamental propositions, from which all explanations of 

 particular phenomena may be deduced. It has been abun- 

 dantly shown, by the experiments of Plateau, Dupre, Van 

 der Mensbrugghe, Terquem, and many others, that every 

 liquid behaves as if its external layer were in a state of 

 tension. This tension is uniform in all directions in the free 

 surface; but its magnitude is modified by contact with the 

 surface of a solid or another liquid. The existence of this 

 surface-tension is the basis of the physical explanation of 

 capillary phenomena. The only assumption it seems necessary 

 to make — namely, that the variations of pressure in the parts 

 of the liquid elevated above the level of the free surface obey 

 the ordinary laws of hydrostatics — seems so evidently natural 

 that it cannot be doubted. 



Commencing with these data, it is easily proved, as if for 

 a stretched membrane, that the intensity of the pressure 

 supported by a meniscus is given at any point in the meniscus 



by the expression p=+H(p- + ^J, where E. and W are the 



principal radii of curvature at that point, and H is the 

 uniform tension across a linear centimetre in the surface. 



The constancy of the angle of contact of a liquid with a 

 given solid also follows easily from the assumption of a 

 surface-tension ; and thus armed we are able to explain any 

 capillary phenomena. 



In the January number of this Magazine, Prof. John Le 

 Conte attempts to refer to two fundamental principles the 

 explanation of several cases of the apparent attractions and 

 repulsions of small floating bodies. He refers these phe- 



