54 MEMBRANE PERMEABILITY AND DRUG ACTION 



permeability of collodion membranes, chitin and of the 

 sulphur bacterium Beggiatoa ndrabilis. Two instances of 

 a collodion membrane are illustrated, one with a large 

 pore size and the other with a small pore size. It will 

 be seen that where pore size is large the permeability 



TABLE VI 



THE PERMEABILITY OF TWO COLLODION 

 MEMBRANES TO DIFFERENT MOLECULES 



The values are of PM^^. Membrane (a) is 

 relatively permeable, and membrane (b) rel- 

 atively impermeable 



falls off inversely as the square root of the molecular 

 weight, and equation (6) is obeyed to a first approxi- 

 mation. On the other hand, where the pore size is small, 

 the permeability falls off much more rapidly than is in- 

 dicated by equation (6). For membranes having a large 

 pore size it is clear that to a first approximation the 

 permeability may be calculated, provided that the per- 

 meability of the membrane to two or three other sub- 



