Spreading of Fluids on Glass. 51 



It thinned in places, and the thicker parts contracted into a 

 number of bead-like drops which then slowly moved towards 

 one another and fused until finally one or a few drops 

 remained, the rest of the plate being apparently but not really 

 free from acid. On lifting the cover even for a moment the 

 drops at once expanded again, and the layer again proceeded 

 io contract when the cover was replaced. 



The question therefore arose whether acetic acid would 

 spread if water vapour were rigorously excluded. By a 

 simple device drops were placed on the glass plate without 

 1 if tin o- the cover, and no flattening occurred. But though 

 the drop remained where it was placed without visible 

 change, there had in fact spread from it over tlie plate an 

 invisible film. In other words, the single pure chemical 

 substance acetic acid behaves on glass as does a single 

 chemical substance on water; from it a film spreads to form 

 a primary composite surface, and if any secondary spreading 

 of the drop itself occurs it is due to an impurity — in this 

 <;ase water. 



It is not possible to measure the surface tension of a solid 

 in the simple way applicable to fluids, but the existence of an 

 invisible film of acid covering the surface can be proved by 

 the fall in static friction which it produces. 



The static friction of a clean glass plate measured in the 

 way described in the last paper was found to be 0*92 gramme 

 per gramme weight of the watch-glass. A small drop of 

 acetic acid was placed on the plate near one corner. It 

 remained apparently unchanged, but the friction of the 

 general surface was found to have fallen to 0'47 gramme 

 per gramme weight. The surface was therefore covered by 

 an invisible film of acid. In the complete absence of aqueous 

 vapour, pure acetic acid forms a primary composite surface 

 on glass with the excess acid gathered into a lens just as, for 

 instance, oleic acid or benzene will form a primary surface 

 in tensile equilibrium with a lens on clean water. 



Tripropylamine, just distilled to a constant boiling-point, 

 exhibits precisely similar phenomena. A drop placed near 

 a corner of a plate lowered the static friction over the whole 

 surface to 0*25 gramme per gramme weight of the watch- 

 glass. 



When water is present in the acetic acid itself, or as vapour, 

 the acid behaves on glass just as does an impure chemical 

 substance on water. A drop spreads to form a secondary 

 composite surface. The concentration of water needed to 

 upset the equilibrium of a drop of pure acid ami induce 

 secondary spreading must be incredibly small. A drop o( 



E 2 



