Theory of Surface Forces. 



475 



ventured briefly to quote it, although the question of the size 

 of atoms lies outside the scope of the present paper. 



Another matter of great importance to capillary theory I 

 will only venture to touch upon. When oil spreads upon 

 water, the layer formed is excessively thin, about two 

 millionths of a millimetre. If the layer be at first thicker, it 

 exhibits instability, becoming perforated with holes. These 

 gradually enlarge, until at last, after a series of curious trans- 

 formations, the superfluous oil is collected in small lenses. It 

 would seem therefore that the energy is less when the water 

 is covered by a very thin layer of oil, than when the layer is 

 thicker. Phenomena of this kind present many complications, 

 for which various causes may be suggested, such as solubility, 

 volatility, and — perhaps more important still — chemical hetero- 

 geneity. It is at present, I think, premature to draw definite 

 physical conclusions ; but we may at least consider what is 

 im'plied in the preference for a thin as compared with a 

 thicker film. 



Fig. 13. Fig. 14. Fig. 15. 



1 



The passage from the first state to the second may be 

 accomplished in the manner indicated in figs. 13, 14, 15. We 

 begin (fig. 13) with a thin layer of oil on water and an 

 independent thick layer of oil. In the second stage 

 (fig. 14) the thick layer is split into two, also thick in com- 

 parison with the range of the cohesive forces, and the two 

 parts are separated. In the third stage one of the component 

 layers is brought down until it coalesces with the thin layer 

 on water. The last state differs from the first by the substitu- 

 tion of a thick film of oil for a thin one in contact with the 

 water, and we have to consider the work spent or gained in 

 producing the change. If, as observation suggests, the last 

 state has more energy than the first, it follows that more 

 work is spent in splitting the thick layer of oil than is gained 

 in the approach of a thick layer to the already oiled water* 

 At some distances therefore, and those not the smallest, oil 

 must be more attracted (or less repelled) by oil than by water. 

 The reader will not fail to notice the connexion between this- 

 subject and the black of soap-films, investigated by Profs. 

 Remold and Pucker. 



2 L2 



