138 Dr. G. Bakker on the 



H = Available energy, total departure of the law of Pascal 

 per unit of surface, constant of Laplace or surface- 

 tension. 



j)x = Hydrostatical pressure in a point of the capillary layer 

 in a direction perpendicular to its surface. 



p T = Hydrost. pressure in a direction parallel to the surface 

 of the capillary layer. 



1 C 2 - 1 C 2 1 P 2 1 C 2 



P» = y] P^ l ip T =A p T dh; px'=p\ pjlh*\p^ = -& 1 p T dK 



§ 2. The radius of curvature of a capillar// layer and 

 the equation of Kelvin. 



The thickness of a capillary layer that limits a spherical 

 liquid mass of measurable curvature may be neglected with 

 respect to the radius of the liquid mass, and it is therefore 

 indifferent whether we consider Rj, R 2 , or R. 



Quite otherwise is it on the contrary if the value of R 

 lies e. g. between one micron and the minimal value of the 

 radius of the spherical liquid mass. In a preceding article 

 I found that the minimum value of the radius of a liquid 

 drop is of the same order of magnitude as the thickness of the 

 capillary layer*. 



If Rj and R 2 are resp. the radii of the spheres which 

 limit the considered spherical capillary layer, we put 



R = — -q~- and call R the radius of the drop. The matter 



within the sphere of radius Rj should be considered as a 

 liquid. If we call pi the pressure in the liquid mass of the 

 drop, p v the pressure of the vapour which limits the drop, 

 H the surface-tension, and R K a value between R x and R 2 , 

 Kelvin found as is known: 



m m 



*-»•* K K (1) 



Meanwhile R K has for very little drops a rather complicated 

 signification. If for a point in. the capillary layer we call 

 p y the pressure in a direction perpendicular to the surface 

 (radial), and p T the pressure in a direction parallel to the 

 surface, we have: 



t=-- ( ^> w 



in which dh represents the differential of the normal to the 



* Phil. Mag. March 1909, p. 346. 



t G. Bakker, Phil. Mag. April 1908, p. 422, formula (17). 



