THE THEORY OF OSMOTIC PRESSURES. 159 



Thus in the case of the pleura we seem to have a mem- 

 brane which is very imperfectly semi-permeable. It is per- 

 meable to salts, but presents rather more resistance to their 

 passage than to the passage of water. Hence on injecting 

 •5 per cent. Na CI solution into the pleural cavity water 

 passes from the pleural fluid into the blood, until the per- 

 centage of sodium chloride in the fluid is raised perceptibly 

 above that in the blood plasma. The limit of the resistance 

 of the pleural membrane to the passage of salt is, however, 

 soon reached, and then salt passes from pleural fluid into 

 blood ; but in every case this passage is from a region of 

 higher to a region of lower partial pressure. Hence at 

 a certain stage of the experiment we find a higher percentage 

 of salt in the pleura than in the blood-vessels, although 

 the total amount of salt in the pleural fluid is less than 

 that originally put in, or, in other words, salt has been 

 absorbed. 



We have already seen that the effective osmotic pressure 

 of a substance, i.e., its power of attracting water across a 

 membrane, varies inversely as its diffusibility, or as the 

 permeability of the membrane to it. What then will be 

 the effect supposing that on one side of the membrane we 

 place some substance in solution to which the membrane 

 is impermeable ? 



We will suppose that A and B both contain 1 per cent. 

 Na CI, but that B contains in addition some substance x to 

 which the membrane is impermeable. Since the osmotic 

 pressure of B is higher, by the partial pressure of x, than 

 that of A, fluid will pass from A to B by osmosis. But the 

 consequence of this passage of water will be to concentrate 

 the Na CI in A, so that the partial pressure of this salt in 

 A is greater than in B. Na CI will therefore diffuse from 

 A to B with the result that the former difference of total 

 osmotic pressure will be re-established. Hence there will 

 be a continual passage of both water and salt from A to B, 

 until B has absorbed the whole of A. This result will 

 be only delayed if the osmotic pressure of A is at first 

 higher than B, in consequence of a greater concentration 

 of Na CI in A. There may be at first a flow of fluid 



