136 Dr. G. Bakker on the 



If we consider the thermic (kinetic) pressure as inde- 

 pendent of the direction, we may assume the difference 

 between the hydrostatic pressures in a point of the capillary 

 layer to be equal to the difference of the cohesions both in 

 directions resp. perpendicular and parallel to the capillary 

 layer. If p y and /> T are the pressures in a point of the 

 capillary layer respectively in directions perpendicular and 

 parallel to the surface of the capillary layer, I have called 

 py—]> v the departure from the law of Pascal at the considered 

 point. The capillary constant of Laplace thus becomes the 

 integral of the expression }>y—pr, or, if p s and ]> T represent 

 average values, we have for a plane capillary layer 



H=(p,.-^.)r. 



in which f represents the thickness of the capillary layer. 

 (For a plane capillary layer p K =p K = ordinary vapour-pres- 

 sure.) By integration of the expression /^—p T Hulshof and 

 Bakker calculated the capillary constant of Laplace. Bakker 

 deduced the complete expression of Rayleigh, and has given 

 also an elementary proof of the exactness of the conception 

 of Fuchs*. As p T at a definite temperature (about |T K ) has* 

 the value zerof, we have for that temperature: 



u _ capillary constant 

 vapour pressure 



We may thus, at least for the mentioned temperature, con- 

 sider this quotient as the value of the thickness of the plane 

 capillary layer. 



If we consider such a large surface of the body, that the 

 total mass of the capillary layer is unity, we call its surface 

 S and consider Sf=r as the specific volume of the capillary 

 layer if it is plane. For the spherical capillary layer (see 

 above) the thickness is expressed by (R 2 — ."Ki): we represent 

 it again by f. It will easily be understood that in this case: 



For the cylindric capillary layer on the contrary we have 

 again i?=Sf, in which S belongs to the radius B= —~ — -. 



* H. Hulshof, Koninkl. Akail v. Wetensch. at Amsterdam, 29 Jan. ] 900. 

 G. Bakker, Zeitschrift f. phijs. Chem. xxxiii. p. 499 (1900) ; and Phil. 

 Mag-, for Dec. 1906, pp. 568 & 569. See Ravleigh, "'On the Theory of 

 Surface Forces II.," Phil. Mag-. Feb. 1892, formula (22). 



t G. Bakker, Phil. Mag. Oct. 1907, p. 522, and Zeitschr.f.phys. Chem. 

 i.1905, p. 359/ 



