on Static Friction. 39* 



be in part consumed in producing this new setting of the 

 molecules. 



There is fortunately direct evidence available to prove that 

 irreversible changes in molecular configuration are produced 

 when clean glass faces are forced over one another. The 

 original interface is preserved at the junction of the thin 

 flakes, torn off one solid, with the other solid, and these 

 flakes are found to be doubly refractive. 



Lubricated Faces. 



The effect of the depth of the layer of lubricant must be 

 taken account of. No attempt was made to measure this,. 

 but three stages were distinguished — the film, the smear,. 

 and complete flooding. A film is a layer completely invisible,, 

 of depth insufficient to give Newtonian colours and almost 

 certainly of the order of 1 /jl/jL. A smear is a visible but thin 

 layer. In complete flooding the watch-glass moves in a 

 pool. 



The difference between the film and the two other states 

 of tiie surface is more than one of mere depth of the layer. 

 We know from experiments with fluids that when a layer of 

 one fluid spreads upon another, the surface energy is at first 

 a function of the depth of the layer. Thus, when olive oil 

 spreads on clean water, the surface tension is a function of 

 the quantity of oil per unit area until this exceeds a certain 

 limit at which two independent interfaces are formed, that 

 of oil-air and that of oil-water. Over the range of varying 

 tension the properties of the surface, such as its chemical 

 potentialities, its electric change, and its mechanical tension,, 

 are a function of the thickness of the film on the surface, the 

 chemical composition of the material composing the film and 

 the fluid on which it lies, and the temperature. Surfaces of 

 this kind, whose properties depend upon the interaction of 

 two kinds of states of matter, have been called by one of us 

 " composite surfaces " *. 



The experiments described in this and the following paper 

 prove that in the " film" stage of lubrication we are dealing 

 with a composite surface. 



The feature of a composite surface which is of most 

 importance in lubrication, is that the energy per unit of mass 

 of the film is a function of position on the axis of the 

 normal. Two consequences follow — the film resists tan- 

 gential displacement, and therefore has tenacity, and, since 



* A soap-bubble is composed of two composite faces placed back to 

 back, cf. Proc Roy. Soc. A. lxxxvi. p. 009 (1912). 



