510 EDMUND NEWTON HARVEY 



Three general theories, based on Van't Hoff's view of the driv- 

 ing power in osmosis, have been advanced to account for the 

 passage of substances through membranes : — the filter theory, the 

 solution theory, and the chemical combination theory. The 

 filter theory regards a membrane as a molecular sieve. Whether 

 it is permeable to a given substance will depend on the relative 

 sizes of the molecules of the substance and the interstices of the 

 membrane. A study of artificial precipitation membranes has 

 afforded considerable evidence against such a simple explana- 

 tion. According to the second theory a substance must dissolve 

 in the membrane in order to pass through, and according to the 

 last theory the substance must combine with the membrane 

 before it ma\ pass. 



J. Traube, ('04, '08, '09, '10) on the other hand, regards the 

 driving force in osmosis, as an 'Oberflachendruck,' later called 

 'Haftdruck,' measured by the tendency of the substance to in- 

 crease or decrease the surface tension of the solvent. The mem- 

 brane separating two phases is not the important thing but the 

 difference in surface tensions, the phase of lowest surface tension 

 tending to pass through the membrane into that of greater sur- 

 face tension. Traube admits that the Haftdruck of the mem- 

 brane may also be a determining factor ('10, p. 533). 



At present there are two general views as to the classes of sub- 

 stances which may diffuse into cells. These are based largely 

 on conceptions regarding the nature of the cell membrane and 

 the physical or chemical process by which a substance may pass 

 it. The results obtained by different methods of investigation 

 and their interpretation have been in many cases conflicting. 



Inasmuch as my studies on the permeability of cells show in 

 the cl earest manner the existence of two distinct classes of alkalies 

 with respect to their ability to enter, a brief statement of the 

 opposed views may not be out of place. 



Overton and Hoeber classify substances into lipoid-soluble 

 and lipoid-insoluble. The former are found to enter cells very 

 rapidly, the latter not at all when tested by the plasmolytic 

 method (p. 512). Yet the lipoid-insoluble substances are just 

 those which we know by analysis and microchemical tests to 



