40 Mr. W. B. Hardy and Lieut. J. K. Hardy 



the potential energy of • the molecules composing it is a 

 function of their position, the film is not completely fluid 

 even though it be formed from material which is a fluid 

 when in mass at the same temperature and pressure. This 

 is true of composite surfaces of fluids as well as of solids. 



Owing to this defect in fluidity composite surfaces are 

 capable of seizing, and the static friction of such surfaces is, 

 in our opinion, due to this fact. 



By seizing is meant the capacity for offering resistance to 

 slip when both faces are at rest. Though it is due to the 

 operation of the same forces of attraction as bring about 

 cohesion, resistance to slip and cohesion are not identical. 

 Any internal surface of a fluid may be considered to be 

 formed by bringing together two fluid faces which cohsre, 

 but there is no resistance to slip gained thereby. Composite 

 faces resist slip because of their defect in fluidity. 



If the matter stopped here, if, that is to say, the defect 

 in fluidity were due solely to the surface energy being a 

 function of the thickness of the film, static friction between 

 composite plane faces would be purely an edge phenomenon 

 and would vanish if the area of the faces was infinite. The 

 way in which the energy of an interface depends upon 

 chemical constitution, the fact, already noted, that it is 

 closely related to the polarizability of the molecules, proves 

 that the surface energy is a function of the orientation of 

 the molecules as well as of their position on (he axis of the 

 normal, and it is to this that we may look for the source of 

 the static friction of composite surfaces as well as of clean 

 faces. There is, indeed, no final distinction between com- 

 posite faces as defined and clean faces, for there is an 

 orientation ot the molecules at the surfaces of any fluid or 

 solid, and this skin, which has its own peculiar configuration, 

 constitutes, as Gibbs pointed out, a separate phase. 



The distinction between the film and the smear or flooded 

 states of a surface is now seen to be not merely one of degree 

 but one of kind. This follows not only from what has been 

 just said, but also from the way in which the surface energy 

 varies with the thickness of the film. The well-known 

 phenomenon of the grey and black area of soap-bubbles, 

 and the fact that a layer of fluid of small but sensible 

 depth when spread on another fluid or on a solid, is not in 

 equilibrium, but breaks under the influence of surface forces 

 int© a film and thick sheets or lenses which are in tensile 

 equilibrium with the film, proves that the surface energy is a 



