166 THEORY OF COLLOIDAL BEHAVIOR 



It is very interesting that this film formation of gelatin on 

 collodion membranes occurs regardless of the pH of the solution. 

 It is, therefore, not necessary that the gelatin (or protein) be 

 ionized to form a film on collodion membranes. 



Aside from the electrical charges, the osmotic pressure of the 

 solution seems also to have an effect on the stability of the col- 

 loidal solution. We shall see that the Donnan effect demands also 

 that the osmotic pressure be influenced in a similar way by the pH, 

 the valency, and the presence of salt as is the P.D. The depress- 

 ing effect of the salt on the difference of osmotic pressure inside 

 and outside the micellae may possibly be of more importance in 

 the precipitation of colloidal suspensions than the depressing 

 effect of the salt on the electrical charge of the micellae. This is 

 indicated by the fact that the difference in the depressing effect 

 of NaCl and Na 2 SO 4 is greater for osmotic pressure than for P. D. 



6. THE ORIGIN OF THE ELECTRICAL CHARGES OF LIVING CELLS 



AND TISSUES 



In his first paper on the theory of membrane equilibria Donnan 

 suggested that the membrane potentials postulated by his theory 

 might contribute towards an explanation of the action of nerves 

 and even of electrical fish. In 1911 the writer suggested to Dr. 

 Beutner that he investigate the P.D. between such organs as 

 apples, or leaves of the rubber plant, and water, instead of the P.D. 

 of muscles or nerves, which had usually been used by physiologists 

 for this purpose. In these experiments Dr. Beutner made the 

 important observation that the P.D. between the surface of an 

 apple or a leaf was a maximum when the bounding liquid was pure 

 water, while the P.D. was depressed when a salt was added to 

 the water, the depressing effect on the P.D. increasing with the 

 concentration of the salt. 1 MacDonald 2 had observed a similar 

 phenomenon, namely, the increase in P.D. between nerve and 

 surrounding salt solution with increasing dilution. Donnan's 

 theory was not known to us and we were not able to give an 

 explanation of the depressing effect of salt on the P.D. 



A search was made for those substances in the cortex of an 

 apple or leaf which might be responsible for these peculiar con- 



, J. and BEUTNER, R., Biochem.-Z., vol. 41, p. 1, 1912. 

 2 MACDONALD, J. S., Proc. Roy. Soc., vol. 67, p. 310, 1900. 



