PASSAGE OF WATER AND DISSOLVED SUBSTANCES 151 



from the pleural fluid into the blood, until the percentage 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 dift'usibility, or as the permeability of the 

 membrane to it. What, then, will be the effect if 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. NaCl, 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 NaCl in A, so that the partial pressure of this salt 

 in A is greater than in B. NaCl 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 NaCl in A. 

 There may be at first a flow of fluid from B to A, but as soon as the 

 NaCl concentration on the two sides has become the same by diffusion, 

 the power of x to attract water from the other side will make itself 

 felt, and this attraction will be proportional to the osmotic pressure 

 of x. We shall have occasion to discuss a specific instance of this case 

 when dealing with the mechanism of absorption of fluid by the blood- 

 vessels from the connective tissue spaces. 



A more familiar example is afforded by the process known as 

 dialysis. Many animal membranes, all of which are colloidal in 

 character, and others such as vegetable parchment, while freely 

 permeable to salts, are impermeable to dissolved colloids. If, therefore, 

 a fluid containing both colloids and crystalloids in solution, e.</. 

 blood-serum, be enclosed in a tube of vegetable parchment, which is 

 hung up in a large bulk of distilled water (Fig. 26), all the salts diffuse 

 out, and if this be frequently changed, we obtain finally a fluid within 

 the dialyser free from salts and other crystalloid substances, but 

 containing the whole of the colloidal proteins originally present. 



Thus the transference of fluids and dissolved substances across 



