PASSAGE OF WATEK AND DISSOLVED SUBSTANCES 135 



Thus the transference of fluids and dissolved substances across membranes 

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

 the diffusion coefficient of the solutes and the permeability of the membrane. 

 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 be passed. 

 (WROBLESKI.) 



membrane vary astheir 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. 



Bayliss has drawn attention to certain other factors which may determine permanent 

 equality of distribution of a salt on the two sides of a membrane permeable to the salt. 

 Congo red, which is a compound of an indiffusible colloid acid with sodium, be placed 

 an osmometer which is immersed in water, a certain osmotic pressure is developed, 

 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 fluids with different percentages of salt. The numbers 

 indicate the litres to which each gramme molecule of the salt is diluted. Apparently 





