358 Mr. A. M. Worthington on the 



s 



the self-attraction of B, then each fluid will be condensed at 

 the common surface, which will therefore be in a state of 

 pressure. Now, as Maxwell has pointed out*, when this is 

 the case the surface must be unstable, for its extension in- 

 volves an evolution of energy being aided by the molecular 

 forces, and the surface will therefore tend to enlarge itself by 

 puckering and replication. It is also probable that this 

 condition would involve what we call chemical action. If, on 

 the other hand, the mutual attraction between A and B is less 

 than the self-attraction of A, and also is less than the self- 

 attraction of B, then each fluid will be in a state of tension 

 near the bounding-surface, which will therefore be in stable 

 equilibrium, and the experimental surface-tension will be the 

 sum of the two sets of forces. 



There remains the case in which one fluid is condensed and 

 the other rarefied at the surface. Here the two sets of forces 

 are of opposite sign, and the surface will be stable or unstable 

 according as the algebraic sum corresponds to a tension or to 

 a pressure. A liquid in contact with its own vapour is, as we 

 have seen, an instance of the former case f . 



18. When, on the other hand, only one of the substances 

 is a fluid, the other being a solid, it is only the tension or 

 pressure of the former that can produce any motion or that 

 enters into experimental determinations; for forces normal to 

 the solid are balanced by the resistance of the solid, and forces 

 within the solid tangential to the surface cannot, owing to 

 the rigidity of the solid, cause a motion of its parts. 



Now in the case of the surface of contact of air and dense 

 solids, such as glass or a metal, there can be no doubt, as 

 we have seen, that the air is condensed on the solid, and that 

 the surface-force within this fluid is a pressure, and from this 

 we are forced to conclude that whenever the liquid-angle of 

 contact between a liquid and a solid in air is acute (or 0°), 

 the liquid also is condensed against the solid, and that, of 

 the three surface-forces concerned, the greatest is the surface- 

 pressure within the liquid in the neighbourhood of the solid, 

 as is evident from a glance at fig. 8, which represents a 

 section normal to the surfaces in question in the neighbour- 

 hood of the common line of contact 0. And it will be per- 

 ceived that the spread of a liquid over a solid, as for instance 

 of a drop of alcohol over glass, is not quite the same pheno- 



* Encyclopcedia Britannica, art. " Capillarity." 



t The latter case, perhaps, corresponds to liquids which mix without 

 chemical action ; for although, as Maxwell remarks, no case of puckering 

 has been observed, yet it is not evident that the phenomenon is one which 

 could be observed or distinguished from diffusion. 



