490 Sir William Thomson [Jan. 29, 



not go tlirongli the whole process again ; the result is obvious. 

 Thirty times the excess of the sum of the surface-tensions of the two 

 liquids separately, above the tension of the interface between them, 

 is equal to the work done in letting the two bodies come together 

 directly over the supposed area of thirty square centimetres. Hence 

 the interfacial tension per unit area of the interface is equal to the 

 excess of the sum of the surface-tensions of the two liquids separately^ 

 above the worh done in letting the tioo bodies come together directly so as 

 to meet in a unit area of each. In the particular case of two similar 

 bodies coming together into perfect contact, the interfacial tension 

 must be zero, and therefore the work done in letting them come 

 together over a unit area must be exactly equal to twice the surface- 

 tension ; which is the case we first considered. 



If the work done between two different liquids in letting them 

 come together over a small area, exceeds the sum of the surface- 

 tensions, the interfacial tension is negative. The result is an 

 instantaneous puckering of the interface, as the commencement of 

 diffusion and the well-known process of continued inter-diffusion 

 follows. 



Consider next the mutual attraction between a solid and a liquid. 

 Choose any particular area of the solid, and let a portion of the sur- 

 face of the liquid be preliminarily shaped to fit it. Let now the 

 liquid, kept for the moment rigid, be allowed to come into contact 

 over this area with the solid. The amount by which the work done 

 per unit area of contact falls short of the surface-tension of the liquid 

 is equal to the interfacial tension of the liquid. If the work done per 

 unit area is exactly equal to the free-surface tension of the liquid, the 

 interfacial tension is zero. In this case the surface of the liquid when 

 in equilibrium at the place of meeting of liquid and solid is at right 

 angles to the surface of the solid. The angle between the free sur- 

 faces of liquid and solid is acute or obtuse according as the interfacial 

 tension is positive or negative ; its cosine being equal to the inter- 

 facial tension divided by the free-surface tension. The greatest 

 possible value the interfacial tension can have is clearly the free- 

 surface tension, and it reaches this limiting value only in the, not 

 purely static, case of a liquid resting on a solid of high thermal con- 

 ductivity, kept at a temperature greatly above the boiling-point of 

 the liquid ; as in the well-known phenomena to which attention has 

 been called by Leidenfrost and Boutigny. There is no such limit to 

 the absolute value of the interfacial tension when negative, but its 

 absolute value must be less than that of the free-surface 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- 

 surfiice tension, the liquid runs all over the solid, as, for instance, 

 water over a glass plate which has been very perfectly cleansed. If 



