51.4 Dr. G. Bakker on th 



complete capillary layer. The surface-tension in the black 

 spot can also be very well equal to the tension H in the 

 complete capillary layer. 



It may not be superfluous to remark that, though the 

 surface-tension in the black spots may have the same value 

 as in the complete capillary layer, the tension is nevertheless 

 a quantity of somewhat other signification, the ordinary 

 constant of Laplace being the surface-tension in a complete 

 capillary layer. 



§ 2. Tlte Hydrostatic Pressure in the Capillary 

 Layer and in the Black Spot. 



If we consider again the liquid-film of fig. 1, we may 

 easily demonstrate that for low temperatures, for instance 

 near the melting-point, the pressure parallel to the surfaces, 

 which limit the film, may be large and negative. Indeed the 

 part of the film to the right of the plane BB : (perpendicular 

 on AAj) is in equilibrium firstly with an external force at 

 A 1? which we measure as twofold the surface-tension, and 

 secondly with the influence of the part of the film to the left 

 of BBj on the considered part. Between the two capillary 

 layers which limit the film the matter is in the ordinary 

 homogeneous liquid state, and therefore the pressure is equal 

 to the exterior pressure or the vapour-pressure. As near 

 the melting-point the vapour-pressure may be neglected, the 

 pressure between the capillary layers of the liquid may be 

 considered as null. The cohesion and thermic pressure in 

 the interior of the film (between the capillary layers) are 

 therefore nearly equal. On the contrary, in the capillary 

 layers tuhich limit the considered film we must have a force 

 directed to the left, which is in equilibrium with half the 

 force in the strings which bind the strips to the walls of 

 the vessel. The value of this force is therefore the constant 

 of Laplace, H. This force, being outivard relative to the con- 

 sidered part (between BB X and i\ x ), must be reckoned as a 

 negative pressure, which means that the cohesion in the 

 capillary layer parallel to their surface is at low temperatures 

 larger than the the thermie pressure. 



If p 2 denotes the pressure in a direction parallel to the 

 surface, dh denoting the differential of the normal to the 

 surface, the condition of the equilibrium demands : 



2 



+ 2H = (2a) 



2 1 p 2 dh 



Putting 



If 2 



