ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 247 



the positively charged water towards the cathode would be possible. 

 But the clay particles which are ireely movable in the water also 

 exhibit their negative charge by their motion towards the anode. 

 The meaning of the endosmosis was not clear, and for a time Uttle 

 attention was paid to it. It was again described in 1816 for sand 

 filters by Porret,^'* and then also by Becquerel, Armstrong, and 

 Daniell. The phenomenon of electrophoresis was further described 

 in tissue fibers by Faraday, ^^ in starch grains by DuBois Reymond,!'' 

 and in carmin and starch by Heidenhain and Jiirgensen.^^ The 

 first more exact investigations on electroendosmosis were carried out 

 by Wiedemann, ^^ and the first theoretical treatment of this subject 

 approaching the views held toda}^ was furnished by Quincke. ^^ This 

 modern theoretical conception resulted from the observation that 

 water does not always move in the direction of the current but at 

 times moves against the current. In this way Quincke showed that 

 the original idea to the effect that water "was carried along by the 

 current" was not correct. He then proposed the theory of the 

 electric double layer wliich has since proved to be so fruitful. Quincke 

 assumed that at the boundary of fluids with sohds two layers of free 

 electricity, one positive and one negative are found at a very small 

 but finite distance from each other. The one forms a coating adhering 

 fast to the soHd, while the other is in the fluid medium. The latter 

 laj^er can be parallelly displaced along the former by means of a 

 tangentially directed current of electric forces. If the solid be a 

 particle suspended in the fluid, then this displacement is manifested 

 in the motion of the particle under the influence of an electric field. 

 If the solid is a porous sohd in the nature of a diaphragm blocking 

 the path of the water, then, in an electric field, the water will migrate 

 through the pores of the diaphragm in the opposite direction. 



The quantitiative relationships of this theory of these phenomena 

 were developed by Helmholtz.-*' 



>^ R. Porret, Gilb. Ann. 66, 272 (1816). 



15 M. Faraday, Exp. Res. Nr. 1562, 1838. 



16 E. Du Bois-Reymond, Berl. Ber. 1860, p. 895. 



1^ E. Heidenhain u. Jurgensen, Arch. f. Anat. u. Physiol. 1860, p. 573. 

 18 Wiedemann, Ann. d. Phys. u. Chem. 87, 321 (1852). 

 '3 Quincke, Ann. d. Phys. u. Chem. 113, 513 (1861). 



20 H. V. Helmholtz, Ann. d. Phys. u. Chem. 7, 337 (1879) ; Ges. Abhandlungen 

 I, p. 855 (1882). 



