494 Albert P. Mathews 



r 



molecules, of the form / e * , as the potential of two mass 



, r 



points; and he goes on to say that this formula was based on 

 two assumptions, namely that the attraction of two molecules 

 is inversely as the square of the distance, and second, that the 

 universal medium absorbs the lines of force. He thus assumes 

 an absorption by the ether of the attraction, rather than by a 

 molecule at a distance "r." In his paper published in 1903, 

 in which the real molecular volume, the value of "6," is no 

 longer considered constant and in which he has revised his 

 formula for the isotherm in so important a manner, he does 

 not specifically reraise the question of the penetration of 

 matter by the cohesive attraction. 



In a succession of papers like those of van der Waals' 

 which represent a progressive succession of ideas, mutually 

 conflicting ideas may, not unnaturally, be found. Thus, in 

 his paper on the thermodynamic potential and capillarity, 1 

 a limiting intermediate layer of rapidly changing density is 

 supposed to exist between the saturated vapor and the liquid, 

 and the existence of such a layer would seem to the writer 

 to presuppose that the radius of attraction at least in this 

 layer must be several molecular diameters. 



On the other hand, the following statement 2 (p. 121) is 

 not entirely reconcilable with this view, and would be so 

 only if the layers of which he speaks are at least equal in 

 thickness to the distance the cohesive force extends. He 

 supposes the cohesive pressure to be exerted only by the 

 surface layer, but he states that exactly the same formulas 

 are obtained if the fluid be considered to be made up of a series 

 of layers of molecular dimensions. "If we consider the gas in a 

 cylindrical vessel of constant area and divided into horizontal 

 layers, the lowest attracts the next higher," etc. The sum of 

 all partial amounts of work will be the same as if one considered 



1 Van der Waals: "Theorie thermodynamique de la Capillarite," Archives 

 Neerlandaises des Sciences exactes et naturelles, 28, 121 (1895). 



2 Van der Waals: "Die Continuitat des gasformigen u. flussigen Zustandes," 

 Leipzig, p. 126 (1899). 



