on Static Friction. 45 



increases. In both acids and alkalies the activity of the fluid 

 increases as the molecular weight increases. 



If these several relations are confirmed by more extended 

 investigations, the case "hard to explain" of lubrication 

 diminishing as the quantity of lubricant increases will arise, 

 but, when the bulk of the measurements given above were 

 made, the extreme sensitiveness of the faces to minute 

 traces of water was not appreciated by us (see the next 

 paper). The figures given in the table were obtained 

 under comparable conditions, but they are not free from 

 the suspicion of the influence of water. The sample of 

 butyric acid almost certainly contained a trace of water. 

 Acetic acid in the complete absence of water- vapour gave 

 the values : — 



Weight of watch-glass. 



Film. 



Flooded 



46*95 grammes. 



0-47 



0-36 



170*2 



: 64 



0-58 



The film, however, is subject to intense evaporation, due- 

 to the rapid current of dry air. The maximum of lubrication 

 was therefore probably not reached. Acetic acid frozen on 

 the plates gave the values : — 



Flooded 



Weight of watch-glass. Film. (i. e. sensible 



layer of ice). 



65-25 grammes. 0'23 0*23 



The sample of tripropylamine was specially purified, and. 

 the values given in the table were obtained in the complete 

 absence of water. The vapour-pressure of this substance is 

 low, and the values therefore are probably close to its 

 maximum of lubrication. 



The evidence, so far as it goes at present, is against the 

 conclusion that lubrication is a function of the quantity of 

 lubricant on the plate when the lubricant is a single pure 

 chemical substance, and when its viscosity is not very 

 great. 



The fluid called "paraffin" is the rectified ''paraffin" of 

 the British Pharmacopoeia. It was somewhat uncertain in 

 its effects, behaving as though it were a mixture of active 

 and inactive substances. 



