1920] on Problems of Lubrication 71 



The esters occupy a quite unexpected position. The simple 

 aliphatic esters are much worse lubricants than their related acids and 

 alcohols. The ring esters, on the contrary, are better lubricants 

 than are their related acids (e.g. ethyl benzoate and benzoic acid). 



Perhaps the most interesting substances are the hydroxy acids 

 with OH and COOH groups. This conjunction produces a remark- 

 able increase in the lubricating power of a chain compound (lactic 

 acid and ricinolic acid), and almost destroys lubricating action in 

 the case of the ring compounds (salicyclic and benzylic acids). 



In the ring compounds the replacement of hydrogen decreases 

 lubricating power in the case of X, : 0, or 'COOH, and increases it 

 in the case of other groups in the order C 2 H 5 ZCH ZOH. 



The effect of a second group of the same or of a different kind 

 is to decrease the effect of the first. Compare, for instance, toluene 

 with xylene ; catechol, quinol, and cresol with phenol ; and methyl 

 cyclohexanol with cyclohexanol. The simpler the group the more 

 effective it is. Compare cymene with toluene or xylene, and benzyl 

 alcohol with phenol. 



When the atoms are disposed with complete symmetry about a 

 carbon atom, the result is a very bad lubricant, as we see in carbon 

 tetrachloride and the alcohol penterythritol C(CH 2 OH) 4 . 



It will be noticed that no ring compound is a good lubricant. 

 Even cholesterol, with the molecular weight 366, is no exception. 



The group SH acts much as OH, thiophenol C 6 H 5 SH and benzyl- 

 hydrosulphide C 6 H 5 .CH 2 SH resembling phenol and benzyl alcohol 

 respectively. 



Concerning one matter, and that the most fundamental, some 

 conclusion must be come to, even though it be upset later. What is 

 friction due to ? The " Encyclopaedia Britannica " is in no doubt as 

 to this. Friction, it says, is due to inequalities of the surface. This 

 conclusion cannot, I think, be accepted. Why, if it be true, should 

 clean burnished faces of glass or bismuth refuse to slide over one 

 another ? It does not even accord with such simple facts as we now 

 know. For instance, the friction of an optical face of glass was 

 found to be the same as that of ordinary plate glass within the limit 

 of accuracy aimed at. And both the optical face and ordinary plate 

 were found to give higher values than ground glass. 



The subject cannot be fully discussed here, but I think we may 

 conclude with some confidence that the friction both of lubricated 

 and of clean faces is due to true cohesion — to the force, that is, 

 which binds together the molecules of a solid or of a fluid. If there 

 were no seizing, there would be no friction. The function of the 

 lubricant is to diminish the capacity for seizing by saturating more 

 or less completely the surface forces of the solid. In some cases it 

 seems to abolish it completely so that static friction vanishes. 



The subject of lubrication is of interest to the engineer, but it 

 is of perhaps more interest to the physicist, for it offers a means of 



