THE THEORY OF OSMOTIC PRESSURES. 157 



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, 

 C0 3 passes into the hydrogen bottle more quickly than 

 hydrogen can pass out into the C0 2 bottle, so that a dif- 

 ference of pressure is created between the two bottles, and 

 the rubber bulges into the C0 2 bottle. We might, in the 

 same way, conceive of a membrane which permitted the 

 passage of dextrose more easily than that of urea. With 

 such a membrane, experiments conducted in the same way 

 as Dr. Barlow's, would lead to diametrically opposite re- 

 sults. 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 dia- 

 phragm, the osmotic flow was in the reverse direction, 

 and an enormous pressure was set up on the alcohol side of 

 the diaphragm. 



Here we have a possible clue to the "explanation" of 

 many 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, very possible that the 

 processes here may be exactly analogous to Graham's 

 atmolysis, and that the bladder may represent a perfected 

 form of Graham's india-rubber bag. 



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 ques- 

 tion on the two sides of the membrane. Great stress is 

 laid by Heidenhain and his pupil Orlow 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 pointing indubitably to an active 

 intervention of living cells in the process. This argument 



