70 Mr. W. B. Hardy [Feb. 27, 



It will be seen that static friction is a function of the molecular 

 weight of the lubricant ; and in a simple chemical series of chain 

 compounds, such as fatty acids and alcohols or paraffins, a good 

 lubricant will be found if one goes high enough in the series. But 

 it is not a simple function. The friction, for instance, rises sharply 

 in moving from CHCl 3 to CCl 4 and from phenol to catechol and 

 quinol. The influence of molecular weight is over-shadowed by the 

 influence of chemical constitution. 



In some simple chemical series the relation appears to be a linear 

 one. Examples are paraffins ; the series benzene, naphthalene, 

 anthracene. 



The relation of lubricating qualities to viscosity broadly resembles 

 that to molecular weight. In a simple chemical series lubrication 

 and viscosity change in much the same way with molecular weight ; 

 but that there is no fundamental relation between viscosity and 

 lubrication is shown by the following figures : — 



Carbon tetrachloride 



Chloroform 



Acetic acid 



Octyllic acid 



Benzene 



Toluene 



Benzyl alcohol 



Fluidity of the lubricant has no constant significance. The 

 curves for acids, alcohols and paraffins show no break where, with 

 increasing molecular weight, the lubricant becomes a solid at the 

 temperature of observation. Compare also benzene, naphthalene 

 and anthracene, menthone and menthol, thymol and carvacrol. 



Perhaps the most unexpected result is the distinction between 

 ring and chain compounds. The simple ring compounds benzene, 

 naphthalene and anthracene show the linear relation to molecular 

 weight, and the values are much the same as those for paraffins of the 

 same molecular weight. The similarities, however, end here, for any 

 change in the molecular structure produces opposite effects according 

 as it takes place in a chain or ring. Thus a double bond decreases 

 the lubricating action of a ring compound, but increases that of a 

 chain compound. As examples, compare naphthoic acid with double- 

 bonded oxygen, with naphthalene, menthone with menthol, cyclo- 

 hexanone with cyclohexane, benzoic acid with benzene. As examples 

 of double-bonded carbon, compare cinnamic ester with hydro-cinnamic 

 ester, di-pentene, having two unsaturated carbon atoms, with menthol 

 and cyclohexane. Also the more saturated cyclic compounds are 

 better lubricants than the less saturated ring compounds.. 



When a ring or chain are joined, as in butylxylene, the result is a 

 better lubricant than either. 



