ELECTROKINETICS • 377 



metals. They cannot usually be reversed by the hydrogen ion, 

 requiring special ions of nearly always high valency. 



Stability. — The precipitation of colloidal metals, the floccula- 

 tion of albumin, the coalescence of suspended droplets (the 

 "breaking" of an emulsion), the coagulation of blood, and the 

 agglutination (clumping together) of bacteria — in brief, nearly 

 all colloidal precipitation phenomena — are, at least in the major 

 aspects of their behavior, due to a reduction in the surface 

 potential of the particles. (It should be emphasized again that 

 the potential and not the charge is responsible.) 



Usually, it is potential which is primarily involved in deter- 

 mining stability. Burton says that mutual electric repulsion 

 {i.e., f potential) is wholly responsible, certainly in the case of 

 metals. Freundlich, Pauli, and others, without actually dis- 

 senting from this viewpoint in its fundamental aspects, attribute 

 stability to a protective envelope of ions. The distinction is 

 slight — a potential is recognized in both cases; but in the one 

 (a protective ionic covering), the particles may strike in their 

 random movement but do not adhere; while from the other 

 viewpoint, they are kept from striking by their mutual electric 

 repulsion. 



Kruyt and deJong lay emphasis on hydration (page 147) 

 which very likely plays a prominent role in the stability of 

 proteins, though Kruyt says that it is not sufficient. 



Whatever the nature of the environment of colloidal particles 

 may be, when it is removed, the particles come into contact, 

 adhere, and settle out. 



IN THE LIVING WORLD 



Electroendosmosis. — The passage of water through living 

 membranes is an important event in the life of an organism. 

 It is often neglected in studies on permeability where attention 

 is usually centered on the passage of salts. The passage of water 

 through a membrane under the influence of an electric potential 

 is electroendosmosis. It may be the force responsible for the 

 transference of water through the living membrane. S. Mudd 

 has shown that when a mammalian membrane (mesentery and 

 pericardium of dogs, rabbits, and human beings), bathed in a 

 dilute buffer solution, is traversed by an electric current, liquid 

 is caused to stream through the membrane toward the cathode 



