52 Mr. W. B. Hardy on the 



acid standing quietly on the surface at once spreads it the 

 cover to the chamber be slightly lifted for a moment whilst 

 the current of dry air is pouring in. 



For reasons to be given presently, it is important to note 

 that these observations on the behaviour of acetic acid were 

 made at 18° C. 



Not all fluids will spread to form a primary composite 

 surface on water, and this is true for glass. At 18° C. 

 neither castor oil nor B.P. k< paraffin" formed such a surface 

 when drops were placed on a plate. 



When more than one drop of acetic acid is present on a 

 plate at I8 C C, the existence of an invisible film about them 

 is manifested in a curious way. Drops which are not more than 

 one or two centimetres apart attract one another. They 

 become oval in outline, the long axes being directed towards 

 one another, and slowly move across the plate until they 

 meet and fuse. The edge of the plate acts like another drop,, 

 a single drop tends to move to the edge and cling there, but 

 the edge does not attract it so strongly as does another 

 drop. The experiment is a simple one, but very striking. 

 The invisible film is nowhere greater than 1 micron in 

 thickness, yet it is capable of pulling large drops of fluid 

 along. The stress per unit area of transverse section must 

 be enormous. 



The reason for the movement of the drops is probably to be 

 found in the evaporation due to the current of dry air. The 

 film everywhere is thinned by this, and the tension of the 

 composite surface therefore greater than what it would be if 

 the space above were saturated with the vapour of acetic acid. 

 The evaporation is continually made good by the drops which 

 are as continually feeding the film. The surface tension in 

 a line joining two drops will therefore always be lower than 

 elsewhere. Why are the drops not pulled away instead of 

 towards each other ? .The answer can only be that on the 

 general surface of the plate the layer is so thin and so closely 

 applied to the solid face as to have lost fluidity. It is 

 practically a non-contractile solid. Between the drops it is 

 thick enough to preserve some degree of fluidity and it 

 contracts, pulling the drops together. This explains the fact 

 that drops more than 1 to 2 centimetres apart do not attract 

 each other at 18° 0. 



There is much of interest in considering how the water 

 vapour acts when it causes the drop of acetic acid to spread 

 over the primary composite surface. The most obvious 

 suggestion is that it is condensed on to the composite sur- 

 face where, by diluting the film of acetic acid, it raises the 



