1920] on Problems of Lubrication 67 



cloth, or glass faces which have been simply exposed to the air, are in 

 point of fact not clean ; they are highly lubricated with a film of 

 matter derived from the cloth or condensed from the atmosphere. 

 This "grease" film is of invisible thinness. It is probably of the 

 order of one fx jx in thickness, that is to say, one millionth of a 

 millimetre. It can be removed by soap and water, which in turn 

 must be removed by a stream of water, and the plates dried in clean 

 air out of contact with solids. The film re-forms quickly — very 

 quickly in London air, and less quickly in the country. A " grease " 

 film also creeps over a cleaned glass face from ordinary solids with 

 which it may be in contact. Still, when due precautions are taken, 

 and they are many, it is possible to get a glass face which seems to 

 be really clean. 



The first property of such faces is that their fricticn, one for the 

 other, is very high ; indeed it is impossible to make them slip past 

 one another. One glass plate may be forced past another, but true 

 slipping does not take place, they tear at the point or points of 

 contact. It is easier in short to disrupt the actual substance of glass 

 itself than to get the surfaces to slip over one another. Clean glass 

 faces '"seize" when they touch. 



AVhen chemical substances are tested ;<s lubricants on clean glass 

 faces a remarkable fact emerges — namely, that some are quite neutral 

 in that they do not alter the resistance to slip in the least ; such are 

 water, alcohol, benzene, strong ammonia. Other substances have 

 some lubricating action great or small. That is to say, they decrease 

 the force needed to produce slipping ; such are the alkalies, trimethy- 

 lamine and tripropylamine, the fatty acids, e.g. acetic acid, and the 

 paraffins. Those fluids which act as lubricants are not neces- 

 sarily fluids of any considerable viscosity, indeed a high viscosity 

 is compatible with the absence of any true lubricating action. Thus 

 glycerine facilitates the slipping of clean glass on clean glass only 

 when it is present in quantity sufficient to float the surfaces apart. 

 On the other hand, acetic acid and tripropylamine, substances of low 

 viscosity, are admirable lubricants. 



Xo fluid amongst those tested has been found to raise the friction 

 of clean glass faces. A fluid either is neutral or decreases friction to 

 a greater or less extent. The property of increasing the friction of 

 glass faces which neutral fluids, such as water, possess, is due not 

 to their action on the glass, but to the fact that they interfere with 

 the effect of the invisible grease film. Water on an ordinary glass 

 face acts as an anti-lubricant ; on really clean glass it is " neutral." 



All solid faces, however, do not distinguish chemical substances 

 into those which are " neutral " and those which possess lubricating 

 properties. Nearly one hundred substances have been tested on 

 burnished faces of bismuth, and in every case some decrease of 

 friction was observed. 



A comparison of the lubricating action of simple chemical sub- 



F 2 



