364 PROTOPLASM 



a mathematical expression for Quincke's suggestion. The 

 electric surface is known as the Helmholtz double layer. 



Numerous mathematical and theoretical hypotheses have been 

 advanced in explanation of the electric layer on the surface of 

 colloidal matter, but these are not needed for a simple interpreta- 

 tion of it. Let us first consider a glass capillary tube in which 

 there is water flowing under the influence of an applied electro- 

 motive force. Quincke and Helmholtz assumed that the inner 

 wall of the tube is lined with negative charges which adhere to 

 the wall (Fig. 159). Adjoining them are positive charges, equal 

 in number and lying within the water. The negative charges 



+ 



\ 



Glass wall—^^ 

 + +y?TT + + + + + -l. + + + + + + -«■+ + -»• 



H/crfenz^r—^-- 



^i^T- + + + +±±±± + ±±±±± ± ± ± ± ± ±\ 



k -, ,- >r, > r; , , , , - > > , , , , , > > > > , > > ~> > , , s , , , , , , ! > > 1 1 1 1 1 ) I > 1 1 1 1 i 1 1 1 1 1 1 1 1\ 



Fiu. 159. — The Helmholtz double layer in a glass capillary tube filled with water. 



adhere to the glass wall and cannot move; the positive charges 

 are free to move, and, being positive, they travel toward the 

 negative pole of the applied electromotive force, carrying the 

 water, of which they are a part, with them. The moving positive 

 charges in the water glide by the stationary negative ones on the 

 glass. 



The positive charges are H+ ions, and the negative ones are 

 OH- ions. (It appears that the H+ and OH" ions are sub- 

 sidiary here to the H+ and HCOs" ions in ordinary water.) 

 Helmholtz could not have anticipated this, as ions were unknown 

 in his time. Water, being a weak electrolyte, is slightly ionized. 

 The glass becomes negatively charged apparently because of 

 preferential adsorption of negative (OH") ions. The moving 

 water is positive, because each OH" ion adsorbed by the glass 

 leaves one H+ ion — its mate — free in the water. The positive 

 H+ ions, being free to move, are attracted by the negative pole 

 of the electric field. The speed at which the water travels 

 through the tube under the influence of an external electromotive 

 force depends on the potential of the current applied and on the 

 potential of the Helmholtz layer. This latter is, in part, deter- 

 mined by the distance 5 between the two layers of negative and 

 positive ions. The dielectric constant, or the electric ionizing 



