[ 509 ] 



XLIX. On the Theory of Surface/Forces. II. 

 By G. Barker *. 



§ 1. Observations of Isaac Newton. 



IF films of liquids, such as oil-films on water or films o£ a 

 solution of soap in water, become continually thinner 

 and thinner, there appear suddenly, as every one knows, 

 on the thinnest places o£ the brilliant surface circular 

 black spots. These spots gradually enlarge so that the 

 surface of the film seems to be perforated with holes. This 

 observation was made first by Isaac Newton. 



I will demonstrate how this phenomenon can be explained 

 in a theory of surface-forces, in which the capillary layer is 

 considered as a gradual transition of the liquid phase to the 

 vapour phase, such as those of Lord Rayleighf by means of 

 the properties of the theoretical isotherms of James Thomson 

 and van der Waals. 



Remold and Riicker \ have made the very important dis- 

 covery that the black spot, always formed before an undis- 

 turbed film of soap solution breaks, has a uniform or nearly 

 uniform thickness of about eleven or twelve micro-millimetres, 

 while the thickness of the remaining parts of the film exceeds 

 fifty micro-millimetres. The sharp boundary of the black 

 spots is also a consequence of the relative great difference 

 between the thickness of the spots and the remaining parts 

 of the film. Let us now consider a liquid membrane, the 

 breadth being equal to the unit, lying between two solid 

 strips supported on the right and the left by cords stretched 

 in a vessel containing nothing but water vapour (see fig. 1). 



2H 



$ 



Fiff. 1, 



sir 



The liquid film cannot be in equilibrium unless the strips are 

 bound to the walls of the vessel §. When thus the thickness 

 of the film exceeds sufficiently twofold the thickness of the 

 capillary layer, the tensions in the strings, which bind the strips 



* Communicated by the Author. 



f " On the Theory of Surface Forces, II." Phil. Ma^. Feb. 1892, p. 209. 

 \ Proc. Roy. Soc. June 21, 1877 ; and Trans. Roy. Soc. April 19, 1883. 

 § Phil. Miff, for December 1906, p. 563. 



Phil. Mag. S. 6. Vol. 1-1. No. S2. Oct. 1907, 2 M 



