on Static Friction. 47 



in an interesting way.' Friction was lowered when the 

 plate was flooded with the spirit, but rose to the "clean" 

 value when the fluid was completely dried off. The point or* 

 interest is that the full value was not restored until the layer 

 of fluid had been reduced by evaporation to well past the 

 Newtonian colour stage. 



Actual values in grammes are : — 



Clean 49; flooded 4b; Newtonian colours seen about 45; 

 colours vanished 46; and one hour later 49. 



The question now arises whether the difference between 

 active and inactive substances is one of degree or one of 

 kind. The final answer must be left to further investigation 

 with more refined apparatus. All that can be said now is 

 that the inactive fluids were inactive for the lightest a^ well 

 as for the heaviest watch-glass used. On the other hand, an 

 insensible film of an active fluid lubricates the surface for the 

 heaviest watch-glass used, and in the case of tripropylamine, 

 oleic acid, castor oil, and paraffin just as effectively as for 

 the lightest glass. The insensible film formed by spreading 

 from a drop of tripropylamine under the influence of surface 

 forces gave for instance the following values : — 



w • ,, c . , i Threshold value 



Weight of watch-glass. . 



° ° per gramme weight. 



Flooded ... 46*95 grammes. 0'24 gramme. 



Film 46*95 „ 0*25 



170-21 „ 0-26 



Temperature 10° C. 



Summary. 



One of us has shown that the variation of the surface 

 energy at an interface between two fluids, and of a compositn 

 surface is closely related to the chemical constitution of the 

 substances concerned. The inference to be drawn is that 

 the work done in forming the interface or the composite 

 surface is done by chemical forces. In the theory of 

 capillarity as developed by Young and Laplace cohesive 

 forces are taken to be, like gravity, incapable of saturation, 

 and unlike gravity only in being of very short range. It 

 cohesive forces are chemical they are capable of saturation, 

 or, as a chemist would put it, of neutralization, and the film 

 -of matter on a composite surface may be said to reduce the 



