134 PHYSIOLOGY 



ence will be most marked in the layers of fluid nearest the membrane. The 

 result, therefore, of the unequal diffusion of the two substances is to upset the 

 previous equality of osmotic pressures. The layer of fluid on the B side of 

 the membrane will have an osmotic pressure greater than the layer of fluid 

 in immediate contact with the A side of the membrane, and there will thus be 

 a movement of water from A to B. Hence if we have two equimolecular and 

 isotonic solutions of different substances separated by a membrane permeable 

 to the solutes, there will be an initial movement of fluid towards the side of 

 the less diffusible substance. 



We have an exact parallel to this in Graham's familiar experiment, in 

 which a porous pot filled with hydrogen is connected by a vertical tube with 

 a vessel of mercury. In consequence of the more rapid diffusion outwards of 

 the hydrogen than of atmospheric air inwards, the pressure within the pot 

 sinks below that of the surrounding atmosphere and the mercury rises several 

 inches in the tube. 



We must therefore conclude that, even when the two solutions on either 

 side of the membrane are isotonic, there may be a movement of fluid from 

 one side to the other with a performance of work in the process. In fact, 

 osmosis may occur from a fluid having a higher towards a fluid having a lower 

 osmotic pressure. If, for example, equimolecular solutions of sodium 

 chloride and glucose be separated 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 

 atmospheres' 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 comparable 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 diffusibility 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 an 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 



* 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. 



