Spreading of Fluids on Glass. 53 



tension and the drop. of acid is pulled out to a plate. Of 

 course, the same result would be produced it' it diluted the 

 drop of acid and lowered the tension o£ one or both faces, but 

 the quantity of water operating would seem to be too small 

 to produce a sensible change in a large drop of acid. 



The suggestion that the water vapour condenses on to 

 the primary composite involves a paradox, namely that a 

 vapour can condense on to a surface whilst raising its surface 

 tension. At first sight this looks like a spontaneous increase 

 in free energy. A parallel case can, however, be found. 

 When the area of a primary composite surface of pure oleic 

 acid spread on water is contracted by narrowing the boun- 

 daries, it is possible sometimes to force the surface tension 

 down to a very low level. The surface is then supersaturated 

 with the acid, and it not infrequently changes, apparently 

 spontaneously, the excess acid gathering itself into lenses, 

 and this change is accompanied by a rise in the tension*. 



Returning to the case of acetic acid, the fact that a drop 

 which has spread rapidly contracts when in dry air, points to 

 the aqueous vapour being condensed on io the composite 

 surface and not on to the lens ; for in the latter case the acid 

 is present in mass, and water would not readily distil off from 

 a very concentrated solution of acetic acid. If the water 

 distils off the primary composite surface more readily than 

 the primary acid, we may conclude that it lowers the surface 

 energy of glass less than does the acid; and this is in 

 agreement with the fact that water does not, and acetic acid 

 does, lower the static friction of glass. 



When a layer of sensible thickness of a pure fluid such as 

 benzene is formed on water and allowed to thin by evapo- 

 ration, it becomes unstable when the thickness falls to a 

 certain point. The layer then thins in places so as to form 

 primary composite surfaces, and the excess benzene is 

 gathered into a lens-shaped drop or drops. The surface 

 energy is a function of the depth of the layer of benzene, 

 but it is a discontinuous function. At the edge of a drop of 

 a pure fluid which is on water and in tensile equilibrium 

 with a composite surface, a peculiar state must therefore 

 obtain, for at the junction of the drop with the film the 

 instable thicknesses must be somehow bridged over, just as 

 are the instable thicknesses at the junction between the black 

 or grey areas with the rest of a soap-bubble. There is then 

 this difference between the formation of a primary and a 

 secondary composite surface by spreading from a drop : in 



* Proe. Roy. Soc. A. lxxxviii. p. ;>i>() (1913). 



