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II. Note on Static Friction and on the Lubricating Properties 

 of certain Chemical Substances. By W. B. Hardy and 

 Flight Lieutenant J. K. Hardy, R.A.F* 



(From the Goldsmith Metallurgical Laboratory, Cambridge.) 



IN the Philosophical Magazine for February 1918, Lord 

 Rayleigh describes experiments which were undertaken 

 to examine more particularly the well-known fact that a few 

 drops of water wetting the parts in contact will prevent a 

 cup of tea from slipping about in a saucer. A glass carriage 

 with three legs terminating in three feet of hemispherical 

 form was made to slide over a plate of glass or copper. The 

 horizontal pull needed to cause movement was found to be 

 42 grams where the surface was covered by a film of oil 

 estimated as being of the order of 1 micron in thickness. 

 The superposition of a layer of water on the film of oil 

 decreased the lubrication, the threshold value of the force 

 rising to 126 grams, that is a threefold increase, and the 

 effect was the same when the water layer was a film 

 deposited by the breath as it was when the plate was 

 completely flooded. 



Paraffin (lamp) oil gave a similar result, the force needed 

 to bring about slipping being least when the layer of oil 

 was of insensible thickness. Therefore, in Lord Rayleigh's 

 words, the "friction is greater with a large dose than with a 

 minute quantity of the same oil, and this is what is hard to 

 explain." 



We started with the object of clearing up if possible this 

 paradoxical phenomenon, and in a certain sense this limited 

 aim has been attained. The two cases, namely, water on a 

 grease film, and lamp oil used alone, are similar in that the 

 phenomena in both cases are due to chemical heterogeneity. 

 Lamp oil appears to consist of substances of high lubricating 

 power dissolved in a more volatile fluid with little or no 

 lubricating power. In the process of forming a thin film 

 the former are concentrated on the surface by evaporation of 

 the latter. 



It was felt at the outset that no progress would be made 

 unless individual chemical substances were used. Ethyl 

 ether, ethyl alcohol, and benzene were tested, the specimens 

 in each case being supposed to be pure. Each fluid was 

 found to act to some degree as a lubricant, but no consistent 

 fingres could be obtained. This suggested the presence of 



* Communicated by the Authors. 



