152 PHYSIOLOGY 



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 membrane vary as 

 their diffusibilities, and are therefore probably some function of their 



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.) 



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 indiffusible colloid acid with sodium, 

 be placed in an osmometer 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 



