22 POPULAR LECTURES AND ADDRESSES. 



tension to admit of equilibrium at a line of 

 separation between liquid and solid. If minus 

 the interfacial tension is exactly equal to the 

 free-surface tension, the angle between the free 

 surfaces at the line of separation is exactly 180. 

 If minus the interfacial tension exceeds the free- 

 surface tension, the liquid runs all over the solid, 

 as, for instance, water over a glass plate which 

 has been very perfectly cleansed. If for a 

 moment we leave the centre of the earth, and 

 suppose ourselves anywhere else in or on the 

 earth, we find the liquid running up, against 

 gravity, in a thin film over the upper part of 

 the containing vessel, and leaving the interface 

 at an angle of 180 between the free surface of 

 the liquid, and the surface of the film adhering 

 to the solid above the bounding line of the free 

 liquid surface. This is the case of water con- 

 tained in a glass vessel, or in contact with a 

 piece of glass of any shape, provided the surface 

 of the glass be very perfectly cleansed. 



When two liquids which do not mingle, that 

 is to say, two liquids of which the interfacial 



