512 Dr. G. Bakker on the 



Though I cannot comprehend that the phenomenon, dis- 

 cussed by Lord Kelvin, requires the molecular heterogene- 

 ousness of matter, I have nevertheless always considered the 

 idea, whereby capillary attraction is explained as merely 

 Newtonian attraction o£ dense molecules (Laplace, Secchi, 

 Kelvin), as a magnificent idea, that is not to be rejected 

 before the contrary is proved. So I have found* for a 

 medium the following formula for the potential function 

 <j>(r) of the forces between their elements, accepting the 

 Newtonian law of forces between the molecules and assuming 

 the law of dispersion of Boltzmann : 



where \= —^ and r = distance between the elements of 

 ±11 



the volume of the medium; R = constant of the gas; 



T = absolute temperature; D = diameter of the molecules; 



f= constant of the gravitation ; /j, = weight of one molecule. 



The meaning is that the potential energy of the elements of 



the homogeneous medium should be the same as that of the 



molecules of the gas. _i_ 



If fju is sufficiently large, the influence of the factor e r+D 



may be important. Because A, is varying inversely as the 



absolute temperature, we see that for our medium at a 



sufficiently elevated temperature the potential function 



approaches to 



and when, moreover, the medium is sufficiently rarefied the 

 function becomes : 



<P(r) = -(, 



so that the cohesion becomes practically null. We find also, in 

 accordance with the consideration of Lord Kelvin, in his 

 popular Lectures and Addresses (vol. i. p. 59, 1891), that 

 the Newtonian law of attraction between the molecules may 

 be very well in harmony with the properties of the forces 

 between the volume elements of the homogeneous medium, 

 considered in the ordinary capillary theory. 



If my consideration upon the abrupt commencement and 

 the permanent stability of the black spot is exact, the twofold 

 thickness of the complete capillary layer of soap-solution at 



* Ann. der Phys. 4« Folge (1903) p. 216. 



