1886.] 



on Capillary Attraction. 



495 



other. In the three bottles now before you the clear liquid is sul- 

 phate of zinc — in one bottle it has a density less than, in another 

 equal to, and in the third greater than, the density of the suljihide 

 — and you see how, by means of the coloured sulj)liide, all the phe- 

 nomena of drops resting uj^on or floating within a liquid int i which 

 they do not diiiiise 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 



Fig. 9. 



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, suf&ce fully to determine the configuration when the volume 

 Vol. XI. (No. 80.) 2 k 



