Static Friction. 203 



cholesterol was the same for an obvious smear of the solid 

 and for the invisible film left when the smear had been 

 wiped awav by cotton-wool moistened with alcohol. On 

 the other hand, the value of the coefficient for a visible 

 layei of stearic acid deposited from benzene or ether was 

 '20 and for an invisible film *15. I incline to the view that 

 the difference is due to the difficulty of getting rid of the last 

 traces of the solvent from some substances. 



The force which just fails to move the slider immediately it 

 is applied is taken as the measure of the friction. Precision 

 in this matter is needed, because static friction is sometimes 

 a function of the time during which the external force 

 has acted. The significance of this time factor is not yet 

 clear. 



Solid Bismuth, — Surface burnished : some of the fluids 

 used, however, etched the surface so as to expose the 

 crystals : such are the acids formic, acetic, propionic, and 

 valeric, and the sulphur compounds thiophenol and benzyl- 

 hydrosulphide. Temperature 11-14° 0. Measurements made 

 in a current of dry air. Weight of slider 70'5 grammes ; 

 radius of curvature 25*5 millimetres. 



The results given in the column headed Static Friction 

 are the values of the ratio applied force in grammes divided 

 by the weight in grammes of the slider. 



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, as inspection 

 of the charts and curves shows. The friction, for instance, 

 rises sharply in moving from CHC1 3 to CC1 4 and from phenol 

 to catechol and quinol. The influence of molecular weight 

 is overshadowed 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 ; and, making allowance for the 

 fact that the ammonia was a solution in water, the series 

 ending with propylamine. 



In the aliphatic alcohols and acids the chain is weighted 

 at one end with the *OH or *OOOH group, and the 

 simple linear relation to molecular weight is disturbed 

 thereby. 



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 



P2 



