PHENOMENA. OF MOTION 63 



concentration of ions on the two sides of the membrane. Let us 

 consider the simplest illustration of the equilibrium between NaR 

 and NaCl represented in our table on page 60 where the original con- 

 centration is NaR : NaCl =1:1 and equilibrium is established when 

 33 per cent NaCl passes from (2) to (1). 

 The schematic representation would be: 



Equilibrium 



In this instance all charges are mutually satisfied excepting those of 

 Cl and R. The slowly diffusing anion R is opposed to the rapidly 

 diffusing anion Cl so that a difference of potential must arise at the 

 boundary surface. In the cases hitherto described, the membrane 

 itself is the seat of the difference in potential. The conditions are 

 quite different if the membrane acts only as a bounding surface, that 

 is, if it is not equally permeable for all ions. In this case, the uni- 

 versally present " contact potential," existing in two contiguous 

 salt solutions, is . modified by the aforementioned property of the 

 membrane. 



Finally, we must recall another kind of membrane which does not 

 fall into any of the previous categories. According to W. NERNST, 

 a film of water upon ether forms a semipermeable membrane for 

 benzol. The experiment is carried out in this way: A pig's bladder 

 is soaked in water; the bladder plays no part, other than to hold the 

 water which forms a partition between ether and ether containing 

 benzol. Here the semipermeability of the membrane depends en- 

 tirely on selective solubility. Benzol is insoluble in water; ether on 

 the contrary has a limited solubility, and as a result ether diffuses 

 through the water to the benzol. Subsequently many such com- 

 binations were devised. They are very extensively distributed in the 

 organism. It is unnecessary to think of complete semipermeability 

 in every case; scattered deposits (fat, lecithin, etc.) may suffice to 

 bring about a partial permeability. 



The membranes of WISTINGHAUSEN depend on this principle of 

 selective permeability. He impregnated with gallic acid salts, 

 animal membranes which then became permeable for fat; by merely 

 washing away the salts the permeability is abolished. Attention 

 should be called to a remarkable observation of ZOTT (cited by 

 H. ZANGGER) which belongs in this chapter. He discovered that a 

 membrane through which sugar has diffused, permitted the passage 

 of gum arabic after it had been moistened with alcohol. 



