ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 279 



The danger of delimiting chemical from physical attractions is 

 obvious from the very nature of these processes, and a serviceable 

 theory is quite evidently one which does not demand any such 

 demarcation. It is under the influence of these ideas that this at- 

 tempt is made to present a unified theory of adsorption potentials. 



82. Hydrodynamic potentials 



It has been previously stated (page 246) that in the study of ad- 

 sorption potentials both methods of electroendosmosis and electro- 

 phoresis can be used. To these we must add two other methods, that 

 of the hydrodynamic current and that of the current of falling 

 particles. 



When water is forced through a porous membrane or through a 

 capillary tube, there arises a potential difference between the two 

 sides of the membrane or between the two ends of the capillary tubes. 

 This phenomenon is the reverse of electroendosmosis. For in the 

 latter water is forced through a diaphragm by the appHcation of a 

 potential difference between its two ends, while in the former the 

 potential difference is produced on forcing water through the dia- 

 phragm. If the two ends of the capillary are led off by means of metal 

 electrodes, a current is obtained as long as the flow of water through 

 the capillary continues. Hence such electric currents are designated 

 as hydrodynamic currents, and the potentials which cause it are called 

 hydrodynamic potentials, '^^ This phenomenon was discovered by 

 Quincke. ^^ A porous clay plate was cemented between the bevelled 

 ends of two glass tubes, or a diaphragm of some powdered substance 

 was tamped into a glass tube. Through such a tube water was forced 

 and platinum electrodes were inserted in the hquid on both sides of the 

 diaphragm. As long as the water was being forced through the dia- 

 phragm an electric current manifested itself whose electromotive 

 force was proportional to the hydrostatic pressure. The E.M.F. 

 was independent of the structure and thickness of the diaphragm, 

 which is in complete analogy with the phenomena involved in elec- 



*^ The German terms "Stromungsstrome" for "hydrodynamic currents" 

 and "Stromungspotentiale" for "hydrodynamic potentials" are perhaps more 

 descriptive than the English equivalents. Transi. 



" G. Quincke, Pogg. Ann. d. Physik. 107, 1 (1859); 110, 38 (1860). 



