42 Mr. W. B. Hardy and Lieut. J. K. Hardy 



0*1 gramme per gramme weight true gliding is completely 

 established. Any small increase in the tangential force 

 beyond the threshold value now causes a slow glide without 

 noticeable acceleration. In other words, the state of the 

 surfaces is such as to dissipate any small increments of 

 energy as heat as fast as they are gained. 



No fluid was found which would increase the facility for 

 slipping beyond this point. It appears to be a true maximum 

 for the kind of surfaces employed. Between this maximum 

 and the minimum of clean surfaces there is a greater or less 

 degree of seizing which must be broken away. The force 

 necessary to effect the break away is too great to be absorbed 

 as heat, and the movement therefore, instead of being 

 a steady glide, is characterized by more or less marked 

 acceleration. 



Films of lubricant can be deposited on the surfaces in. 

 many ways, such as by bubbling the air which comes to the 

 chamber through the fluid (ethyl alcohol, ethyl ether, ben- 

 zene, water, ammonia fortiss., acetic acid) ; by flooding the 

 surface and evaporating off the excess with dry pure air 

 until nothing visible remains (acetic acid, tripropylamine, 

 and triethylamine) ; by flooding the surface, washing off 

 excess with a vigorous stream of tap-water, and draining 

 and drying the plate and watch-glass in the way described 

 earlier (acetic acid, oleic acid, sulphuric acid, castor oil, and 

 paraffin); or by flooding the surface with a very dilute 

 solution in pure dry ether and evaporating off the ether with 

 dry air (oleic acid, c;istor oil). 



Technically, the most beautiful method of forming a film 

 is by taking advantage of the fact, dealt with more fully in 

 the next paper, that a totally invisible film is formed about a 

 drop of some fluids when placed upon a clean glass plate, 

 provided water-vapour is completely excluded. With a sub- 

 stance such as tripropylamine, in order completely to alter 

 the state of the whole surface of the plate it is sufficient to 

 place near one corner a small drop of the fluid. 



It is not permissible to form a film by polishing off excess 

 with " clean " linen since such linen for these purposes is 

 not clean. Simply polishing the surface with clean linen 

 effectively lubricates it by leaving behind an invisible film. 



If the static friction of lubricated surfaces is determined 

 by the variation of the surface energy, we should expect to 

 find it closely dependent upon the chemical constitution of 

 the lubricant. This is the case, and the most unexpected^ 



