254 HYDROGEN ION CONCENTRATION 



zero. The effect of an externally applied electric field upon such a 

 system is but a tangential, gUding displacement of both of the layers 

 constituting the double layer. 



The mechanism of electrophoresis can be depicted as follows. 

 In fig. 28A let a be a small ball which is surrounded by an electric 

 double layer consisting of a solidly adhering negative layer and of a 

 displaceable positive layer. The direction of the lines of force of the 

 external electric field is shown by the arrows. Thus the ball with its 

 fastened negative layer must be displaced to the right and the outer 

 positive layer to the left, so that, for a moment, the state shown in B 

 results. The positive layer which has become defective on the right 

 side regenerates at once, even before the state shown in B is reached, 

 with the help of the positive ions meanwhile brought up by the 

 external electric current. And, Hkewise, the positive ions which were 

 set free at the left end of the outer layer find their electric equivalents 

 in the negative ions brought up by the current, or, they sei^ve to 

 replace the positive ions lost on the way. As it can be readily seen, 

 the entire ball participates in the conduction of the electric current. 



74. The relation of electroendosmosis and electrophoresis to the 



ionic theorj'' 



So much for the purely physical investigations on this subject. In 

 these, the chemical nature of the solution and diaphragm being 

 assumed as given, the chief problems were to correlate the extent or 

 value of endosmosis or cataphoresis with such physical data as the 

 intensity of the current and electric field, the width of the pores etc., 

 and from the numerical values of endosmosis and cataphoresis, under 

 the given conditions of current, electric field etc., to calculate the 

 potential difference between the solid wall and the liquid. We shall 

 now consider the question of the effect of the chemical composition 

 of the sohd wall and of the solution upon the phenomena of en- 

 dosmosis and cataphoresis as well as upon the above potential. At 

 the same time the question arises as to chemical structure of the 

 electric double layer, a question wliich was left open by Hehnholtz. 

 In harmony with our present day theory, we shall at once conceive 

 the double layer as being made up of ionic layers, just as we assumed 

 the double layers of electrode and interphase boundary potentials 

 to be composed of layers of free ions. At this point we take up again 

 our connecting thi-ead with the theory of ionic adsorption which we 

 temporarily abandoned in section 70. 



