30 POPULAR LECTURES AND ADDRESSES. 



and you see how, by means of the coloured 

 sulphide, all the phenomena of drops resting upon 

 or floating within a liquid into which they do not 

 diffuse may be observed, and, under suitable 

 arrangements, quantitatively estimated. 



When a liquid under the influence of gravity 

 is supported by a solid, it takes a configuration 

 in which the difference of curvature of the free 

 surface at different levels is equal to the difference 

 of levels divided by the surface tension reckoned 

 in terms of weight of unit bulk of the liquid as 

 unity ; and the free surface of the liquid leaves 

 the free surface of the solid at the angle whose 

 cosine is, as stated above, equal to the interfacial 

 tension divided by the free-surface tension, or at 

 an angle of 180 in any case in which minus the 

 interfacial tension exceeds the free-surface tension. 

 The surface equation of equilibrium and the 

 boundary conditions thus stated in words, suffice 

 fully to determine the configuration when the 

 volume of the liquid and the shape and dimensions 

 of the solid are given. When I say determine, I 

 do not mean unambiguously. There may of 



