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



by the forces of cohesion in the formation of an interface is; 

 determined by the chemical nature of the substances con- 

 cerned, and that it is Greatest when the molecules of these 

 substances are of a salt type such as esters, acids, or alcohols.. 

 It was then pointed out that such molecules are readily 

 polarizable, and that they would be oriented by the forces 

 acting across the interface * and that this orientation is the 

 cause of contact difference of potential. From this it was 

 inferred that "The surface film (of fluid or solid) must 

 therefore have a characteristic molecular architecture and; 

 the condition of minimal potential involves two terms, one 

 relating to the variation of density, the other to the 

 orientation of the fields of force" of the molecules t. 



The theory of surface forces has since been developed; 

 along these lines in a very striking and beautiful way by 

 Harkins i and Langmuir§. 



Any polarization of the molecules at the surface must intro- 

 duce a factor in the resistance to slip whirh is absent from any 

 resistance there may be to displacement along the normal,. 

 for the former includes the resistance which the molecules 

 may offer to dis'urbance of their orientation, and this might 

 be as great as or even greater than the resistance to displace- 

 ment of the molecule as a whole along the axis of the normal 

 to the interface. 



If this view be correct, the effect of a tangential force- 

 would be to produce a fresh orientation of the molecules at the 

 interface, and this would, amongst other things, alter the 

 contact electrical potential between the taces. In the case 

 of fluids, the new orientation would disappear when the 

 external force was removed, but in the case of solids the new 

 orientation would be irreversible if it exceeded a certain 

 small amount ||, and the work done in causing slip would 



* Proc. Rov. Soc. A. lxxxvi. p. 610 (1912): ibid. A. lxxxviii. p. 312: 

 (1913). 



t Ibid. A. lxxxviii. p. 330 (1913). 



X Journal of the American Chem. Soc. xxxix. p. 354 & p. 541 (1917).. 



§ Ibid, xxxix. p. 1848 (19] 7). 



|| Since the above was written we have come across two lectures by 

 Sir Alfred Ewing, in which he suggests that friction may be due to 

 the attraction between molecules. (" The Molecular Process in Magnetic 

 Induction," Royal Institution of Great Britain, May 1891 : " The Inner 

 Structure of Simple Metals," Journal of the Institute of Metals, viii. 

 1912.) When one face is made to slide past the other " the polar forces 

 continue to act across the plane of sliding, causing first a quasi-elastic 

 turning (of the molecules) ; but when a certain very limited range of 

 movement is exceeded there is dissipation of energy through the original 

 bonds being broken and new bonds established with oscillation of the 

 particles.** 



