210 On Static Friction, 



does not and the latter does lubricate the surface of glass. 

 A drop of each was placed on a clean glass plate a few 

 millimetres apart with striking results. The drop of acid 

 chased the drop of benzene to the edge of the plate where, 

 owing to the characteristic edge repulsion, the latter was 

 split into two. We have, I think, in this observation direct 

 evidence that the forces of attraction operate more strongly 

 between a solid face and a good lubricant than between it 

 and a bad lubricant. 



The better lubricant is more strongly adsorbed by the solid 

 face. Olefines are better lubricants than paraffins, and one 

 of the methods in use for freeing the latter from the former 

 is by taking advantage of the fact that olefines are more 

 strongly adsorbed by a solid. The impure paraffin, in 

 commercial practice, is filtered through a dry powder to 

 clear it of unsaturated subtances. 



Of the pair of fluids acetic acid-water, a drop of the latter 

 pursues strongly a drop of the former on a plate. The 

 surface tension of water is much higher than that of acetic 

 acid, and, since water does not and acetic acid does lubricate 

 glass, the decrement due to interaction with the solid face is, 

 according to theory, less for water. Therefore, in respect of 

 both terms of the right-liLiid member of the equation the 

 advantage lies with the insensible film of water. 



In both of these pairs the result was the same whichever 

 one of a pair was placed first on the plate. Another pair, 

 benzene and acetic acid, gave uncertain results. According 

 to theory benzene should move away and the acid follow, and 

 this usually happened. Sometimes, however, when a drop 

 of acid was placed on a plate on which a drop of benzene 

 already stood, the latter darted away, as though the vapour 

 of acetic acid had lowered the tension of the insensible film 

 of benzene between the drops ; sometimes the drops simply 

 moved away from each other. These complications are to 

 be expected, for we have the tensions of three films to 

 consider — those of the relatively pure films outside each 

 drop, and that of the film of mixed origin between them. 

 If the vapours condense in the last in such proportions as to 

 produce a film of tension less than that of either pure film, 

 the drops will be pulled apart. 



Reasons are given in the earlier paper for believing 

 that an insensible film about a drop is formed always by 

 condensation of vapour and not by direct spreading. 



