ELECTROKINETICS 367 



then points out how it is that an electrically neutral particle 

 can move. He says; 



On the whole, the algebraic sum of the two equals zero, and the center 

 of gravity of the complete system, solid particle, and surrounding posi- 

 tively charged fluid layer taken together cannot be moved by the electric 

 forces which arise from the potential fall in the liquid through which the 

 current passes. However, the electric force will tend to bring about a 

 displacement, relative to each other, of the positively charged fluid 

 layer and the negatively charged particle, whereby the fluid layer follows 

 the flow of positive electricity while the particle moves in the opposite 

 direction. 



Thus the particle, with its adhering layer of negative ions (if it 

 is a negatively charged particle), slips from under the outer layer 



A B 



+ + + ++++ + ++ + ++++ + + + + + + _+ + + + + + -!-+ + + ++ + + ++ + + + + + 



+ - + ~ + - -_ + -_+ -_+- 



-4"+" + --. 

 Fig. 160. — Schematic representations of the diffuse layer of ions surrounding 



colloidal particles. 



of freely suspended positive charges (Fig. 77). In spite of this 

 rather satisfactory explanation, other difficulties arose which led 

 first Billitzer, then Gouy, to advance the hypothesis that the inner 

 layer is closely adherent to the surface of the particle, as Helm- 

 holtz stated, while the outer layer is not a single layer at all but a 

 cushion or cloud of ions. We thus, in another way, escape the 

 difficulty that arises when the colloidal particle is viewed as a 

 neutral condenser made up of two compact layers of charges, 

 equal in magnitude and opposite in sign. The concept of a diffuse 

 ionic cloud surrounding colloidal particles is now generally 

 accepted. The exact arrangement of the ions in the diffuse layer 

 is obviously not known. There may be a cloud with the Helm- 

 holtz double layer more or less clearly marked {A, Fig. 160), or 

 there may be a closely adhering single layer of one electric charge 

 (which determines the sign of the particle) and an adjoining 

 diffuse layer of the opposite charge {B, Fig. 160). 



The charges that constitute the Helmholtz double layer or the 

 Gouy cloud are assumed to be ions. In the case of the electro- 



