PASSAGE OF WATER AND DISSOLVED SUBSTANCKs , ; 7 

 Thus the transference of fluids and dissolved substances across n.,',,,1,, , 

 is determined not only by the osmotic pressure of the solutions, but also b^ 

 the difiuaon coefficient of the solutes and the permeability of the membr 

 This permeability may be of the same character as the permeability of water 

 in which case the rates of passage of the dissolved substances across the 



FIG. 26. Dialyser, consisting of a tube of parchment paper immersed in a vessel 

 through which a constant stream of sterile distilled water can bo passed. 

 (WF.OBLESKI.) 



membrane vary as their diffusibilities, and are therefore probably some func- 

 tion of their molecular weights. On the other hand, the membrane may 

 exhibit a certain attraction for, or power of dissolving, some of the solutes to 

 the exclusion of others, in which case there will be no relation between the 

 diffusibilities and the rates of passage of the dissolved substances. 



In a recent paper Bayliss has drawn attention to certain other factors 

 which may determine permanent inequality of distribution of a salt on the 

 two sides of a membrane permeable to the salt. If Congo red, which is a 

 compound of an indifTusible colloid acid with sodium, be placed in an osmo- 

 meter which is immersed in water, a certain osmotic pressure is developed. 

 On adding sodium chloride either to the inner or outer fluid, there is a fall 

 in the osmotic pressure if time be allowed for equilibrium to be established. 

 At this point it is found that the outer fluid, which is free from dye, contains 

 a larger percentage of sodium chloride than the inner solution of dye. This 

 difference is permanent and is more marked the greater the concentration of 

 the dye salt. In the following Table is given the concentrations of the two 



