86 



internal pressure) must be the same in all directions. Hence a 

 condition occurs in the capillary laver in which tlie external pressure 

 in the direction of this layer p^^ has a quite different value from the 

 pressure normal to this layer p^ i.e. the pressure in the homogeneous 

 vapour and liquid phases. In the surface a tension appeared to exist: 



ƒ< 



The molecular pressure could be easily defined so that the surface 

 tension was in aoreement with the capillary energy determined by 

 Prof. VAN DER Waals by a thermodynamic way : 



ƒ 



in which (> = — and v is the volume for a molecular quantity of 



V 



M grams and 



s = C — ao . 



2 fZ/i^ 4/ dh' 



The two integrals, which must be extended over tKe full height 



of the transition layer, are equal, and this is also the case with two 



corresponding elements so that: 



Qie-2\i] + p,v—iiM) = p,—p^ 



from which immediately follows 



s — 7\^l + p^v = iiM. 



As p^, the pressure in the direction of the capillary layer, has 



the same value in the homogeneous vapour and liquid phases as 



p^, it holds for the quantity 



that // has a constant value both In the homogeneous vapour and 

 liqa'ul phase and in the capillary la>/er. This property leads us at 

 once to expect that it will play an important part especially for 

 kinetic considerations, and that it will express that the number of 

 particles that two arbitrary phases will exchange in the same time, 

 will be equally great. It will, in fact appear that this quantity 

 makes the capillary layer accessible for the considerations developed 

 by Prof. VAN der Waals in his paper on the kinetic significance of 

 the thermod_ynamical potential. Assuming that really in the direction 

 of the capillary layer the pressure p.^ is different from the pressure 

 />,, and besides entirely different in different layers, the neglect of 

 this circumstance will make it impossible to derive the thermodynamic 

 conditions of equilibrium for the capillary layer from kinetic con- 



