OSMOSIS 187 



As water enters, the concentration of the solution is reduced. 

 Turgor is increased, but osmotic pressure is decreased because osmotic 

 pressure is proportional to concentration, and if concentration is 

 changed, the osmotic pressure is changed. This makes it clear 

 why osmotic pressures cannot be measured in terms of turgor. 



As more water enters, the increased turgor opposes osmosis 

 (excess diffusion of the incoming water). The ratio between 

 incoming and outgoing water molecules is decreased. Ulti- 

 mately, a state of equilibrium results, and osmosis ceases. 



Greater concentration of sugar outside the sack will produce 

 excess diffusion in the opposite direction from that thus far 

 considered, and any previously existing pressure within the sack 

 will be relieved. Diffusion in this direction, from within to 

 without, is known as exosmosis, in. distinction from the more 

 usual endosmosis. 



If the solutions on the two sides of the membrane are of the 

 same substance (salt, sugar, etc.) and of like concentrations, no 

 osmosis will take place, and the solutions are said to be isosmotic 

 or isotonic. If they are of different concentrations, then the one 

 of higher concentration is hypertonic, and the one of lower con- 

 centration hypotonic, to the other. Osmosis occurs from a 

 hypotonic solution to a hypertonic solution and is at zero between 

 isotonic solutions. (The story holds strictly where the two solu- 

 tions are of the same kind, e.g., both sugar; but where of different 

 kinds, e.g., salt on one side and sugar on the other, or of mixtures, 

 dissociation and other factors enter in.) 



Kinds of Osmosis. — If pure water is separated from pure 

 water by a water-permeable membrane, and if the temperature 

 of the water on one side of the membrane is raised, the activity 

 of the molecules on that side of the membrane will be increased. 

 More of these molecules will therefore pass through the mem- 

 brane. Thermal osmosis results. 



If pure water is separated from pure water by a membrane 

 permeable to water, and an electric potential difference is estab- 

 lished between the two sides of the membrane, then there occurs 

 a movement of water from the one (electropositive) side to the 

 other (electronegative) side. This is electroosmosis, about which 

 w^e shall have more to say later (page 358). 



Again, we may have a so-called negative phenomenon which 

 exists solely in virtue of the fact that our attention is centered on 



