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



Bt mces on clean faces of glass, and of bismuth, would seem to show 

 that the quality of oiliness is due to some reaction between the 

 substance and the solid face. Much is still obscure, but certain facts 

 seem to be capable of interpretation in no other way. Thus water 

 and ethyl alcohol have no detectable lubricating action On clean 

 glass, whilst both are moderate lubricants for clean bismuth. 



The thickness of the layer needed to lubricate is astonishingly 

 small. It is quite invisible, and probably only one or a very few mole- 

 cules thick. To discuss this adequately would take too long, but the 

 fact may be instanced by an experiment of great beauty. A tiny drop 

 of, say, acetic acid or tripropylamine is placed near one corner of a 

 plate of clean glass 6 cms. square ; nothing detectable by the senses 

 happens, the drop is there and that seems to be all. But the whole 

 surface of the plate has in fact been changed fundamentally. It 

 is now fully lubricated by an invisible film which has spread 

 rapidly over it from the drop. The presence of this film may be 

 detected by measuring the friction or by following the migration of 

 two drops of fluid over the face of the plate. It will be found that 

 the drops attract one another under conditions which point to the 

 cause being the contractility of the invisible film. 



This brings me to the second part of my subject — namely, the 

 relation of lubricating power to chemical constitution. 



In particular experiments with bismuth a slider having a curved 

 surface was applied to a plain surface of metal, both surfaces being 

 highly polished, and the force required to initiate movement was 

 measured. This force measures what is usually called static friction 

 as opposed to the kinetic friction when the surfaces are in relative 

 motion. The static friction was found to be a function of the 

 weight of the slider. Therefore, as a relative measure, the ratio of 

 the weight of the slider to the weight needed to move the slider was 

 used. The results appear in the following Table : — 



Static Friction for clean faces •o, 

 Chain Compounds. 



Methyl 



Ethyl 



Propyl 



Butyl 



Amyl 



Octvl 



Cetyl 



