158 



SCIENCE PROGRESS. 



requires examination. Supposing the two vessels A and B 

 to be separated by a membrane which offers free passage 

 to water, and a difficult passage to salts. Let A contain '5 



n 



A 



B 



per cent, salt solution and B a solution isotonic with a 1 

 per cent. Na CI, but containing only '65 per cent, of this 

 salt, the rest of its osmotic tension being due to other dis- 

 solved substances. If the membrane were absolutely " semi- 

 permeable," water would pass from A to B until the two 

 fluids were isotonic, i.e., until A contained 1 per cent. Na CI 

 (we may regard volume of B as infinitely great to 

 simplify the argument). If, however, the membrane per- 

 mitted passage of salt, the course of events might be as 

 follows : At first water would pass out of A, and salt would 

 diffuse in until the percentage of Na CI in A was equal 

 to that in B. There would now be an equal partial pres- 

 sure of Na CI on the two sides of the membrane, but the 

 total osmotic pressure of B would still be higher than A. 

 Water would therefore still continue to pass from A to B 

 more rapidly than the other ingredients of B could pass 

 into A. As soon, however, as more water passed only 

 from A, the percentage of N a CI in A would be raised 

 above that in B. The extent to which this occurs will 

 depend on the impermeability of the membrane. As soon, 

 however, as the Na CI in A reaches a certain concentration 

 it will pass over into B, and this will goon until equilibrium 

 is established between A and B. Extending this argument 

 to the conditions obtaining in the living body, we may con- 

 clude that neither the raising of the percentage of a salt 

 in any fluid above that of the same salt in the plasma, nor 

 the passage of a salt from a hypotonic fluid into the blood 

 plasma, can afford in itself any proof of an active interven- 

 tion of cells in the process. 



