286 Lord Rayleigh on the 



the Law of Superficial Cohesion. The argument is certainly 

 somewhat obscure ; but as to the character of the " physical 

 foundation " there can be no doubt. " We may suppose the 

 particles of liquids, and probably those of solids also, to possess 

 that power of repulsion, which has been demonstrably shown 

 by Newton to exist in aeriform fluids, and which varies in the 

 inverse ratio of the distance of the particles from each other. 

 In air and vapours this force appears to act uncontrolled ; but 

 in liquids it is overcome by a cohesive force, while the par- 

 ticles still retain a power of moving freely in all directions 



It is simplest to suppose the force of cohesion nearly or per- 

 fectly constant in its magnitude, throughout the minute dis- 

 tance to which it extends, and owing its apparent diversity to 

 the contrary action of the repulsive force which varies with 

 the distance." 



Although nearly a century has elapsed, we are still far from 

 a satisfactory theory of these reactions. We know now that 

 the pressure of gases cannot be explained by a repulsive force 

 varying inversely as the distance, but that we must appeal to 

 the impacts of colliding molecules*. There is every reason 

 to suppose that the molecular movements play an important 

 part in liquids also ; and if we leave them out of account, we 

 can only excuse ourselves on the ground of the difficulty of 

 the subject, and with full recognition that a theory so founded 

 is probably only a first approximation to the truth. On the 

 other hand, the progress of science has tended to confirm the 

 views of Young and Laplace as to the existence of a powerful 

 attraction operative at short distances. Even in the theory of 

 gases it is necessary, as Van der Waals has shown, to appeal 

 to such a force in order to explain their condensation under 

 increasing pressure in excess of that indicated by Boyle's law, 

 and explicable by impacts. Again, it would appear that it is 

 in order to overcome this attraction that so much heat is 

 required in the evaporation of liquids. 



If we take a statical view of the matter, and ignore the 

 molecular movements f , we must introduce a repulsive force 

 to compensate the attraction. Upon this point there has been 

 a good deal of confusion, of which even Poisson cannot be 

 acquitted. And yet the case seems simple enough. For con- 

 sider the equilibrium of a spherical mass of mutually attracting 

 matter, free from external force, and conceive it divided by 



* The argument is clearly set forth in Maxwell's lecture "On the 

 Dynamical Evidence of the Molecular Constitution of Bodies " (Nature, 

 volxi. p. 357,1875). 



t Compare Worthington, " On Surface Forces in Fluids," Phil. Mag. 

 xviii. p. 334 (1884). 



