264 



PHYSIOLOGY OF ALIMENTATION. 



why water moves from a region of lower concentration to 

 one of higher, is not yet entirely understood. The old idea 

 was that the dissolved particles ''attracted" the water. A 

 more correct explanation of the phenomenon based upon 

 differences in surface tension seems to be the following. 

 Liquids (water in this case) are surrounded by a contractile 

 surface film, in consequence of which they always tend to 

 occupy as small a space as possible (that is, tend to become 

 spherical). These surface films, therefore, exert a pressure 

 in a direction toward the centre of the liquid, as shown in 

 Fig. 29, a. The pressure exerted by the diffusion of dissolved 

 particles is evidently opposite in nature to that exerted by 



FIG. 29. 



such surface films, for the particles of a dissolved substance 

 move from the centre toward the surface of a liquid and 

 press upon it. This is -indicated in Fig. 29, b. In the vessel 

 B in Fig. 28, which contains only pure water, we are dealing 

 with surface tension only, which we will represent by P. 

 In the osmotic cell we have this same surface tension, but it 

 is counteracted by the osmotic pressure p. Evidently, there- 

 fore, P>Pp. Or, to put the same in w r ords, the surface 

 tension of the water outside of the cell is greater than the 

 surface tension inside of the cell minus the osmotic pressure. 

 Water passes from the outer vessel into the cell, therefore, 

 because it is squeezed in and not because it is pulled in. 



It is clear that what has been said regarding pure water 



