Lord Rayleigh : Investigations in Capillarity. 3-J7 



contraction of the surface — the tension is the same as that of 

 pure water. 



The next question for consideration is — at what point will 

 an opposition to contraction arise ? The answer must 

 depend upon the forces supposed to be operative between the 

 molecules of oil. If they behave like the smooth rigid spheres 

 of gaseous theory, no forces will be called into play until they 

 are closely packed. According to this view the tension would 

 remain constant up to the point where a double layer com- 

 mences to form. It would then suddenly change, to remain 

 constant at the new value until the second layer is complete. 

 The actual course of the curve of tension deviates somewhat 

 widely from the above description, but perhaps not more 

 than could be explained by heterogeneity of the oil, whereby 

 some molecules would mount more easily than others, or by 

 reference to the molecular motions which cannot be entirely 

 ignored. If we accept this view as substantially true, we 

 conclude that the first drop in tension corresponds to a com- 

 plete layer one molecule tbick, and that the diameter of 

 a molecule of oil is about l'O pp. 



An attractive force between molecules extending to a 

 distance of many diameters, such as is postulated in Laplace's 

 theory, would not apparently interfere with the above reason- 

 ing. An essentially different result would seem to require a 

 repulsive force between the molecules, resisting concentration 

 long before the first layer is complete. In this case the 

 tension would begin to fall as soon as the density is sufficient 

 to bring the repulsion into play. On the whole this view 

 appears less probable than the former, the more as it involves 

 a molecular diameter much exceeding 1*0 pp. 



Explanation of Figures. 



In the figures (and in the tables) there is no relation between 

 the scales of the abscissae representing the densities in the various 

 cases. As regards the ordinates, representing weights or tensions, 

 the scale is the same in all the cases, but the zero point is arbitrary . 

 It may be supposed to be situated on the line of zero densities 

 at a point 4*1 below the starting point of the curve. 



Phil. Mag. S. 5. Vol. 48. No. 293. Oct. 1899. 2 B 



