CAPILLAR Y A TTRA CTION. 2 1 



surface-tension of the liquid is equal to the inter- 

 facial 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 surfaces of 

 liquid and solid is acute or obtuse according as 

 the interfacial tension is positive or negative ; 

 its cosine being equal to the interfacial tension 

 divided by the free-surface tension. The greatest 

 possible value the interfacial tension can have 

 when positive, 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 conductivity, 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 ol the free-surface 



