OF VITAL PHENOMENA 61 



If the surface be convex the surface tension is greater, in- 

 creasing with the convexity. In Fig. 23, m and M are two mole- 

 cules equidistant beneath a plane surface (in) and a convex 

 surface (M). These molecules attract and are attracted by the 



Fig. 23. Scheme showing that convexity increases surface tension and 

 concavity decreases it. 



molecules within the lightly shaded areas, the spheres of molecu- 

 lar attraction. The uncompensated downward attractions are 

 represented by the more deeply shaded areas. The area is larger 

 in the convex film and, hence, the surface tension greater. 



The interior of a drop of a liquid is under a hydrostatic pres- 

 sure due to its surface tension, the pressure being greater the 

 smaller the drop. Imagine a spherical drop bisected by a plane. 

 If the surface tension is unity, the pull of the surface film on 

 the plane = 2T = the hydrostatic pressure on one side of the 

 plane = pT 2 , where p is the pressure. Since pT 2 = 2T, 



2T 2 



p = = — , hence when r is small p is large, or when the 



T 2 r 



radius is small the pressure is large. Thus, the pressure is 



greater in small drops when the surface tension remains the 



same. But it was shown above that the surface tension is greater 



the smaller the drop (the greater the curvature). Hence, the 



pressure is increased in two ways. 



Now suppose the surface film of a drop of liquid that floats 

 in another liquid of equal osmotic pressure to be semipermeable. 

 The surface tension of the drop causes a hydrostatic pressure 

 in its interior and pure water will be squeezed out through the 

 surface film until the increase in osmotic pressure equals and 

 replaces the hydrostatic pressure due to surface tension. Hence 

 surface tension increases osmotic pressure. 



Without knowing the surface tension of protoplasm we cannot 

 estimate the effect of surface tension on the osmotic pressure 



