PASSAGE OF WATER AND DISSOLVED SUBSTANCES 149 



by a peritoneal membrane, the osmotic flow will take place from 

 the fluid having the higher osmotic pressure sodium chloride. * 

 We might compare with this experiment the results of separating 

 hydrogen at one atmosphere's pressure from oxygen at two atmo- 

 spheres' pressure by means of a plate of graphite. In this case the 

 initial result will be a still further increase of pressure on the oxygen 

 side of the diaphragm a movement of gas against pressure taking 

 place in consequence of the greater diffusion velocity of hydrogen. 



So far we have considered only the behaviour of solutions when 

 separated by a membrane, the permeability of which to salts is com- 

 parable to that of water ; so that the passage of salts through the 

 membrane depends merely on the diffusion rates of the salts. There 

 can be no doubt, however, that we might get analogous movements 

 of fluid against total osmotic pressure determined, not by the diffu- 

 sibility of the salts, but by the permeability of the membrane for the 

 salts a permeability which may depend on a state of solution or 

 attraction existing between membrane and salts. We have a familiar 

 analogue to such a condition of things in the passage of gases through 

 an india-rubber sheet. If two bottles, one containing carbonic acid, 

 the other hydrogen, be separated by a sheet of india-rubber, carbon 

 dioxide passes into the hydrogen bottle more quickly than hydrogen 

 can pass out into the carbon dioxide bottle, so that a difference of 

 pressure is created,and the rubber bulges into the carbon dioxide bottle. 

 We might, in the same way, conceive of a membrane which permitted 

 the passage of dextrose more 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 membrane, 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. "j~ 



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

 stance 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 



* In consequence of ionic dissociation of the sodium chloride, a decinormal 

 solution of this salt will have an osmotic pressure nearly twice as great as that 

 of a similar solution of the non-ionised glucose. 



t 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. 



