.48 On Static Friction. 



surface energy by neutralizing to a greater or less extent the- 

 forces at the surface of a solid or fluid. The function of a 

 lubricant is to reduce the energy of the surface, and thereby 

 to reduce the capacity for cohesion and the resistance to slip 

 when two composile surfaces are applied the one to the other.. 

 The function of a lubricant therefore is the opposite to that 

 of a flux. A good deal is to be gained, in our opinion, by 

 recognizing lubrication as a special case of that incomplete 

 chemical reaction characteristic of surfaces in which the law 

 of multiple and definite proportions does not hold. 



Evidence for the orientation of the molecules at an inter- 

 face or on a composite surface which we take to be the 

 source of static friction, is to be found in the fact that- 

 chemical substances whose molecules are by their nature 

 readily polarizable such as those of acids, bases, and esters, 

 produce the greatest changes in surface energy *, and that 

 there is contact difference of potential between the film of a 

 composite surface and the matter on which it lies f- This 

 holds even when no part of the matter concerned can with, 

 certainty be said to be present in mass as in soap bubbles. 

 In these "free films" the films on each face are at 

 an electrical potential different from that of the middle 

 portion J. 



The theory of static friction which seems best to accord 

 with the facts is that it is due to cohesion between the faces.. 

 When a lubricant is present we may consider the friction as 

 operating at an imaginary surface situated in the lubricant 

 parallel to and midway between the solid faces. This sur- 

 face is an interface between two composite surfaces, and 

 may be considered as being formed by bringing these two 

 composite surfaces together. Work is done by the forces of 

 cohesion when the film of lubricant is applied to the solid 

 face to form each composite surface, and the surface energy 

 is decreased by this quantity. A further quantity of work 

 is done when the two composite surfaces are applied to one 

 another with a further change in the surface energy. The 

 static friction is an unknown function of the total change in, 

 surface energy. 



* Proc. Roy. Soc. A. lxxxviii. p. 303 (1913). 

 t Proc. Rov. Soc. B. lxxxiv. p. 220 (1911). 

 t Proc. Roy. Soc. A. lxxxvi. p. 608 C912). 



