PHENOMENA OF MOTION 59 



tain low concentrations; stronger solutions work to a certain extent 

 in the opposite direction. 



The influence of electrolytes on the membranes of the organism and 

 their permeability may depend on different causes. It may, for ex- 

 ample, affect the swelling, and thus, the permeability. Alkalis in 

 general increase the swelling; so also do acids in great dilution, but 

 when concentrated they usually cause shrinking. Chemical changes 

 due to chromic acid, etc., diminish the permeability. 



The influence of electrolytes is not however limited to this rather 

 indirect action. We shall see on page 77 et seq., that differences of 

 potential may develop at the interface between a solid phase and a 

 fluid. Thus, for example, cellulose and wool are negatively charged 

 with respect to pure water. In the case of the majority of animal 

 membranes, most of which are amphoteric, the difference in potential 

 develops only in faintly alkaline or faintly acid water. The pres- 

 ence of salts likewise raises or lowers the difference in potential. 

 The difference in the adsorbability of ions is accounted for also in 

 this manner. Wherever a difference in potential exists, the diffusion 

 rate of water is lowered. Salts, on the other hand, develop a differ- 

 ence of potential in the course of diffusion; in their passage through 

 a membrane, they raise or lower the existing difference in potential. 

 Vice versa, on this account the diffusion rate of salts is affected by 

 the difference in potential existing in the membrane. 



A membrane may thus be the seat of an electromotive force (F. 

 HABER,* GIRARD*), provided that it separates two salt solutions, or 

 a salt solution from water, and that one solution be faintly acid or 

 faintly alkaline. In the first instance the diffusion through the mem- 

 brane will be much impeded, in the latter much accelerated. 



The results of R. BuRiAN* 1 also indicate differences in potential in 

 the ultrafiltration of albumin-salt mixtures; at reduced pressures, he 

 obtained as filtrate a salt solution isotonic with the liquid filtered. 

 If he filtered under increased pressure, the filtrate contained a lower 

 salt concentration than the original solution. F. G. DONNAN* has 

 made an unusually important and fundamental study of membrane 

 equilibria, based upon the osmotic pressure and membrane potential 

 of electrolytes containing a colloidal ion. I shall try to present 

 DONNAN'S ideas without entering upon their mathematical basis. 

 Let R represent an acid colloid, e.g., congo red, which forms a salt 

 with Na, and let the line which separates the colloid electrolyte from 

 water in our diagram be a membrane, impermeable to the colloid. 



(a) Membrane hydrolysis. Let us consider what occurs when the 

 outer water is constantly renewed as described on page 92 et seq., 

 which may be represented in the following diagram: 



Initial condition Terminal condition 



T>TT 



N OH 



water 



water NaOH 



