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Objectioxs to Laplace's Theory of Surface Tension. 



(Abstract.) 



By Arthur L. Foley. 



Laplace's Theory of surface tension attributes the contractile force of 

 liquid films to the attraction of the molecules innnediately beloAv the surface 

 of the liquid for those on the surface, producing a tendency for the surface 

 molecules to move iuto the interior. The magnitude of this force would 

 depend on the curvature of the surface and would be greater at a convex 

 surface than at a flat or concave surface. Conseciuently the rise of water 

 in a capillary tube would be due to the fact that the downward pressure 

 of the film outside the tube is greater than the downward pressure of the 

 film inside the tube. 



This theory does not call for a negative pressure under the film inside 

 the tube. It calls for a positive pressure, but slightly less than the down- 

 ward pressure outside. The liquid then would be forced up the tube by the 

 outside film j)ressure. It would appear then that any variation of the 

 pressure either inside or outside the tube should be followed by a change 

 in the height of the capillary column. Some simple experiments give re- 

 sults that are at variance with the theory. 



Take a long capillary tube wilh its lower end extending some distance 

 into the water and note the height of the capillary column. Drop some soap 

 solution on the water outside the tube and thus lower the tension outside 

 If the liquid is supported by the excess of pressure outside the tube, the 

 height of the capillary colunui should be lessened. On the contrary the 

 height remains constant for some tijne — hours even — until the solution has 

 had time to diffuse into the tube. 



Repeat the experiment this time introducing the soap solution into 

 the capillary tube by means of a very fine capillary tube. The tension in- 

 side the tube being reduced (demanding a reduced pressure inside) and 

 the outer pressure remaining constant, it would seem that the excess of 

 the outside pressure would be increased and that the water should there- 

 fore rise in the capillary. Instead of rising it immediately falls. 



