FIG. 25. 



PASSAGE OF WATER AND DISSOLVED SUBSTANCES 136 



easily than that of urea. The importance of the membrane in determ, 

 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 

 they regard this absorption as point- 

 ing indubitably to an active inter- 

 vention of living cells in the process. A B 

 This argument requires examination. 

 Let us suppose the two vessels A 

 and B (Fig. 25) to be separated by a 

 membrane which offers free passage 

 to water and a difficult passage to salts. Let A contain 0-5 per cent. 

 salt solution and B a solution isotonic 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 

 the 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 betweei 

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

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



* 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-b 

 fishes, for instance, we find a gas which is extremely rich in oxygen ai 

 is said to have been secreted by the cells lining the bladder. It ^'however, pebble 

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

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



