Mr. J. T. Kiley on Capillary Phenomena. 195 



nomena exclusively to the elastic reactions of the tense 

 surface-film, whose form is modified by the proximity of the 

 partly immersed solid bodies, and purposely leaves out of 

 consideration the modifications of hydrostatic pressure. Of 

 course it is evident that these phenomena are primarily 

 produced by the elastic reactions of the capillary surfaces in 

 the neighbourhood of the solid bodies; but to consider the 

 motions as produced by a " superior tension " in one or more of 

 the films, owing to a smaller radius of curvature of the film, 

 is evidently an error of expression. 



The tension is constant in every direction throughout 

 the capillary surfaces; and it is clear that, since for each 

 plate the angle of contact is the same on both sides, the 

 horizontal components of the tensions will balance each other. 

 The movements of the bodies are secondary results of the 

 surface-tension, being produced by the modified hydrostatic 

 pressures on their surfaces. 



In the case of the compound plate of steel and glass, 

 Prof. Le Conte's treatment has evidently led him into an 

 error ; for he says " It is obvious that the tensile reaction can 

 only tend to press the plates together: it cannot produce a 

 motion of translation/'' On the contrary, as I shall show, the 

 effect of the tensile forces is to tend to pull the plates 

 asunder ; but these opposite forces being equal in amount, no 

 movement of the plate ensues. In order to gain a clear 

 physical conception of the reasons for these apparent attrac- 

 tions and repulsions, it is necessary to consider all the forces 

 at work ; and that I propose to do in the following expla- 

 nations : — 



Case I. (fig. 1). Let A and B be two flat plates partially 

 immersed in a liquid which wets both. If the plates be 

 brought sufficiently near each other, the capillary surface QQ' 

 will rise above the ordinary level of the liquid outside the 

 plates. Let MN be drawn in the horizontal surface: if we 

 neglect the atmospheric pressure, the hydrostatic pressure at 

 this level will be zero. Passing upwards the pressure becomes 

 negative, decreasing according to the ordinary law. There 

 is thus a reaction of the nature of a tension between the 

 liquid above MN and the solid surfaces in contact with it. 

 From P to S the hydrostatic tension between the surfaces on 

 the left of A will be balanced by the tension between the 

 surfaces on the right; but from S to Q the tension to- 

 wards the right is unbalanced by any tension towards the 

 left. Since the angle of contact of the liquid with the 

 solid is the same on both sides of A, the horizontal com- 

 ponents of the surface-tensions are equal and in opposite 



