r,6 Mr. W. B. Hardy [Feb. 27, 



It is not sufficient, therefore, to interpose a liquid film between solid 

 faces to get lubrication. Indeed, as the experiment proves, water 

 increases the friction ; it is an anti-lubricant for ordinary faces of 

 glass. 



Is then the quality of lubricant a property of a fluid ? Does water 

 fail to act merely because it does not possess that property to which 

 the name " oiliness " is sometimes given ? Another simple experiment 

 supplies the answer. Instead of a giass plate let us use a plate of 

 ebonite. The glass plate does not readily slip on this. The angle at 

 which slipping occurs is steeper than when a glass plate is used. 

 Now, when the lower half of the ebonite plate is wetted it is found 

 that a glass bottle encounters relatively high friction on the un wetted 

 part, but slips quite freely on the wetted part. Water, in short, is an 

 admirable lubricant for glass on ebonite. 



Here is another plate, picked up at random in the laboratory of 

 the Eoyal Institution. Its composition is unknown. Tested in the 

 same way. water has no detectable influence on the friction between 

 glass and the surface of this plate. 



It will be well to confess at once that these simple experiments 

 raise questions which are as yet without an answer, and that much of 

 what follows concerning them is merely tentative. They seem to 

 establish two things, the first being the curious paradox that a film 

 of fluid introduced between two surfaces does not always decrease 

 friction ; it may, indeed, very much increase it. The second, that 

 the quality of " oiliness," the quality, that is, which enables a sub- 

 stance to act as a lubricant, seems to be not the property of a given 

 fluid, but only of that fluid considered in reference to a particular 

 surface. 



It is necessary at this stage to clear away a possible explanation 

 of the paradox. When two solid faces are separated by a thin film 

 of fluid, capillary forces operate and in certain cases, at any rate, 

 they resist slipping. They will so act, for instance, when the move- 

 ment of the one face past the other increases the area of the free 

 surface of the film. Water has a high surface tension : the capillary 

 forces to which it gives rise are unusually large. Therefore it is 

 pertinent to ask whether, when a layer of water diminishes the 

 facility for the slipping of glass on glass, it is owing to capillary 

 action. A qualitative answer is to be found in the fact that water 

 does in some cases, as when glass is applied to ebonite, increase the 

 facility for slipping ; and Lord Rayleigh furnished the quantitative 

 answer. He calculated the magnitude of the capillary effect and 

 found it negligible compared to the actual friction of glass on glass 

 wetted with water. An appeal to capillary forces of this type will 

 not solve the paradox. 



Some light is thrown upon it when we enquire into the state of 

 the surface of glass whose friction is increased by water. Surfaces 

 of glass 4i cleaned " in the ordinary wav by rabbins: with a dass- 



