366 PROTOPLASM 



Why some colloidal particles preferentially adsorb negative, 

 and some positive, ions is not definitely known. Hypotheses 

 have been advanced; for example, the negative charge is pre- 

 sumed to be due to the high dielectric constant of water; but 

 there are many kinds of particles that are positive in water. 

 Preferential adsorption may rest upon a weak initial charge of 

 the metal, of opposite sign to that of the particle ; thus, a positive 

 metal would adsorb negative ions, and a negative particle would 

 result. More convincing is the hypothesis of relative solubility. 

 If a metal is soluble in water, an environment of its own ions 

 will be formed around it. 



Not only are colloidal suspensions positive or negative, but 

 charges produced by the Dorn effect may be positive or negative. 

 Burton has found that lead shot dropped through water, alcohol, 

 benzene, toluene, and xylene carry down a negative charge; 

 through turpentine and carbon tetrachloride, a positive charge; 

 and through ether, no charge. 



The Helmholtz formula, as given above for electroendosmotic 

 flow, also applies in a modified form to the migration of particles. 

 The formula for cataphoretic migration is 



47n7 



where U is the velocity of the particle in centimeters per second ; 

 and H, the field strength in electrostatic units per centimeter, 

 i.e., the potential gradient, or drop, per centimeter. In the 

 development of this form of the original formula, a number of 

 workers, notably Perrin, have played a part. A comparison of 

 the two formulas reveals that E/l in the one first given for electro- 

 endosmotic flow has been replaced by H. H is the potential 

 gradient, or drop in voltage, per centimeter and is therefore 

 identical with E/l, for E is the total external electromotive force, 

 and I, the distance between the poles. 



The character of the immediate environment of colloidal par- 

 ticles has been the subject of much speculation. Helmholtz 

 thought it to be a compact double layer of charges, but he was the 

 first to point out that an equal number of opposite charges would 

 leave the particle electrically neutral, a condition that would not 

 bring about movement in an electric field. He mentions this and 



