188 PROTOPLASM 



one process while another is predominating; the former appears 

 to be negative because the latter is the stronger and therefore 

 positive. Negative, or anomalous, osmosis has been described 

 as occurring through membranes with pores of different sizes, 

 due to potentials set up. The potential difference set up between 

 the two sides of the membrane causes, presumably, an anomalous 

 flow of water, but it is anomalous only in regard to osmosis due to 

 concentration; the electroosmosis taking place is in accordance 

 with expectations. Ordinary osmosis, due to concentration, 

 does not take place in one direction, because osmosis in the oppo- 

 site direction, due to electric potential, is greater. 



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Fig. 103. — Molecules of pure water on one side, and of water and sugar on the 

 other side of a semipermeable membrane. 



Theory. — Why should water diffuse into a sugar or other solu- 

 tion against pressure? A number of theories have been advanced 

 to answer this question. Let us imagine that 12 molecules of 

 water are striking a membrane, which is permeable to water but 

 not to sugar, along a given line on the pure-water side of the 

 membrane. There will be fewer water molecules striking the 

 membrane along a similar line on the other (solution) side, 

 because sugar molecules are also there (Fig. 103). A certain 

 proportion of the water molecules which strike the membrane 

 will pass through from both sides. As there are more water 

 molecules per unit volume on the pure-water side than on the 

 solution side, more water molecules will enter the solution than 

 will leave it. Consequently, there will be diffusion of water 

 from the outside (pure water) to the inside (solution). This 

 excess diffusion of water is osmosis. 



The now generally accepted theory of osmotic pressure is 

 that based on the fact that a solution is not in equilibrium with its 

 solvent, at the same temperature and pressure. The two may be 

 put into equilibrium by lowering the temperature of the solvent 

 until the vapor pressure equals that of the solution, or one can 

 change the pressure on either the solvent or the solution until 

 equilibrium is established. The tension necessary to put the 

 solution and solvent into equilibrium is the osmotic pressure. 

 Such a comparison between osmotic pressure and vapor pressure 



