Theory of Surface Forces. 511 



touch each other in forming a new state of equilibrium, and 

 because the new film, not containing the densities between C — D 

 and C 1 — D 1 is not complete, it becomes suddenly thinner. 



Although it is possible that the two layers AD and A l D 1 

 precipitate in each other sooner than we have supposed, I 

 believe I have demonstrated that the observation of Newton 

 may be predicted by a theory of the capillary layer, such as 

 the theory of Rayleigh or the theory of van der Waals, where 

 the layer is considered as a continual transition of the liquid- 

 density to the vapoor-density. 



In his popular Lectures and Addresses, vol. i. pp. 8 & 9 

 (1891), Sir William Thomson (Lord Kelvin) says:— "The 

 abrupt commencement and the permanent stability of the 

 black film demonstrate a proposition of fundamental im- 

 portance in the molecular theory : — The tension of the film, 

 which is sensibly constant when the thickness exceeds fifty 

 micro-millimetres, diminishes to a minimum, and begins to 

 increase again when the thickness is diminished to ten micro- 

 millimetres. It seems not possible to explain this fact by 

 any imaginable law of force between the different portions 

 of the film supposed homogeneous, and we are forced to the 

 conclusion that it depends upon molecular heterogeneousness.'" 

 Whereas my considerations are very well applicable to a 

 theory of the capillary layer in which the forces of cohesion 

 are considered as the attractive forces between the elements 

 of volume of the liquid, it seems that the conclusion of Lord 

 Kelvin, where he says that the abrupt commencement and 

 the permanent stability of the black film depend upon 

 molecular heterogeneousness, is not indispensable. Further, 

 the author continues (/. c. ): — " When the homogeneous molar 

 theory is thus disproved by observation, and its assumption 

 of a law of attraction augmenting more rapidly than accord- 

 ing to the Newtonian law when the distance becomes less 

 than fifty micro-millimetres is proved to be insufficient, may 

 we not go farther and say that it is unnecessary to assume 

 any deviation from the Newtonian law of force varying 

 inversely as the square of the distance, continuously from the 

 millionth of a micro-millimetre to the distance of the remotest 

 star or remotest piece of matter in the universe ; and, until 

 we see how gravity itself is to be explained, as Newton and 

 Faraday thought it must be explained, by some continuous 

 action of intervening or surrounding matter, may we not 

 be temporarily satisfied to explain capillary attraction merely 

 as Newtonian attraction intensified in virtue of intensely 

 dense molecules movable among one another, of which the 

 aggregate constitutes a mass, of liquid or solid/ 5 



2M2 



