PASSAGE OF WATER AND DISSOLVED SUBSTANCES 133 



easily than that of urea. The importance of the membrane in determining 

 the direction of the osmotic passage of fluid is well illustrated by Raoult's 

 experiments. When alcohol and ether were separated by an animal mem- 

 brane, alcohol passed into the ether, whereas if vulcanite were employed for 

 the diaphragm, the osmotic flow was in the reverse direction, and an enormous 

 pressure was set up on the alcohol side of the diaphragm.* 



The next point to be considered is the passage of a dissolved substance 

 across membranes, in consequence of differences in the partial pressure of 

 the substance in question on the two sides of the membrane. Stress has been 

 laid by Heidenhain and others on the fact that in the peritoneal cavity, as 

 well as from the intestine, salt may be taken up from fluids containing a 

 smaller percentage of this substance 

 than does the blood plasma, and m 



they regard this absorption as point- 

 ing indubitably to an active inter- 

 vention of living cells in the process. ^ o 

 This argument requires examination. 

 Let us suppose the two vessels A and 

 B (Fig. 25) to be separated by a mem- 



brane which offers free passage to water FIG. 25. 



and a difficult passage to salts. Let 



A contain 0-5 per cent, salt solution and B a solution iso tonic with a 1 per 

 cent. NaCl, but containing only 0-65 per cent, of this salt, the rest of its 

 osmotic tension being due to other dissolved 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. NaCl (we may 

 regard volume B as infinitely great to simplify the argument). If however 

 the membrane permitted passage of the dissolved substances, 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 NaCl in A was equal to that in B. There 

 would now be an equal partial pressure of NaCl 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 

 te other ingredients of B could pass into A. As soon however as more 

 water passed out from A, the percentage of NaCl in A would be raised above 

 that in B. The extent to which this occurs will depend on the impermeability 

 of the membrane. As the NaCl in A reaches a certain concentration it will 

 pass over into B, and this will go on until equilibrium is established between 

 and B. Extending this argument to the conditions obtaining in the 

 living body, we may conclude that neither the raising of the percentage of a 



* Here we have a possible clue to the explanation of some phenomena of cell activity, 

 to which the term * vital ' is often assigned. In the swimming-bladder of fishes, for 

 instance, we find a gas which is extremely rich in oxygen, and the oxygen is said to 

 have been secreted by the cells lining the bladder. It is, however, possible that the 

 processes here may be analogous to Graham's atmolysis, and that the bladder may 

 represent a perfected form of Graham's india-rubber bag. 



