on Static Friction. 43 



fact is that certain fluids have no power of lowering the 

 friction. 



The inactive fluids discovered were : — Ethyl alcohol, ethyl 

 ether, benzene, water, and ammonia t'ortiss. sp. gr. *880. 

 Films of insensible thickness and visible layers were without 

 any influence upon the friction, as also was light and heavy 

 flooding. Finally, these fluids when dried off left the 

 surfaces uncontaminated and the cohesion unchanged. 



It is of course obvious that when a layer of any fluid in 

 mass is maintained between solid faces, the friction will be 

 fluid •friction and the facility for slipping in-finite. Thus, 

 when two glass plates are flooded with water or ammonia and 

 placed face to face, the one plate can be readily slipped past 

 the other since a layer of fluid is maintained between them 

 by capillarity. But a very slight normal pressure suffices 

 to displace such a layer, and the faces then seize. Inactive 

 fluids therefore are lubricants only in this limited sense.. 

 They are inactive in the sense that they have little or no power* 

 of so altering the solid surfaces as to facilitate slipping. 



This statement may be put in another way. The available 

 energy of a composite surface is a function of the thickness 

 of the layer of fluid. If the energy increases as the thick- 

 ness is reduced the layer will resist, displacement. In the 

 case of inactive fluids the variation of the available energy 

 must be such as to confer little or no capacity for resist- 

 ing displacement. The behaviour of ordinary commercial 

 glycerine is interesting in this connexion. 



Glycerine is remarkable in that it can increase the facility 

 for slipping nearly if not quite to the maximum, but only 

 when the surfaces are heavily flooded, when, owing to its 

 viscosity, and in consequence of the rocking forward of the 

 watch-glass, a thick layer of fluid is maintained between 

 the solid faces. A film of glj-cerine had no effect whatever,, 

 thus : — 



Glass 2. Clean : Threshold value 51 grammes. 



Plate 

 Films*. Smear. flooded and 



Heavily 



flooded. 



drained. 



Threshold value ... 51 43 30 6 



Per gr. pressure ... *87 *73 "51 *10 



* Deposited from a '002 per cent, solution in alcohol, the plate being 

 flooded with the solution and dried off without draining- four successive 

 times without change in the threshold value. 



