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OpskCTIONS To LAPLACE’s THEORY OF SURFACE TENSION. 
(Abstract. ) 
By ArtTHuR L. FOLEY. 
Laplace’s Theory of surface tension attributes the contractile force of 
liquid films to the attraction of the molecules immediately below the surface 
of the liquid for those on the surface, producing a tendency for the surface 
molecules to move into 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. Consequently 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 wp the tube by the 
outside film pressure. 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 with 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 column should be lessened. On the contrary the 
height remains constant for some time—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, 1t 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. 
