ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 289 



parent that solid wall-potentials form the connecting link with the 

 oldest known electric phenomenon, viz., friction electricity, which 

 is still the most obscure of all electric phenomena. 



It is still unknown whether in these cases the free electricity is 

 represented by ions or by free electrons, although for substances in a 

 non-metaUic state we must consider the participation of free electrons 

 as highly improbable. No contradiction can be found in Coehn rule 

 to the laws governing the better known relationships. On the other 

 hand to accept it as a universally apphcable law would be far fetched. 

 For it was shown, for instance, that the sign of the charge of water 

 against glass is reversed by the presence of a trace of an aluminum 



TABLE 43 



Water 



Glycerin 



Alcohol 



Acetic acid 



Aniline 



Propionic acid. 

 Ethyl butyrate 

 Amyl acetate. . 

 Chloroform. . . . 

 Ethyl ether. . . . 

 Butyric acid. . . 



Benzene 



Turpentine oil. 



Sign of charge 

 against glass 



+ 

 + 

 + 

 + 

 + 

 -(I) 



+ 



salt, which could certainly have no detectable effect upon the dielec- 

 tric constant of the water. There is also no occasion to speak of the 

 superposition of the established ionic laws upon an universally valid 

 Coehn's rule as of a fundamental principle. Undoubtedly all these 

 phenomena may be explained in a coordinated way. Our present day 

 theories extend only as far as our knowledge of the ionization of the 

 phases permits. For all other cases Coehn's rule may be accepted 

 as being empirically correct to a certain extent. Recently R. Keller" 

 made use of Coehn's rule for the explanation of biologically im- 

 portant processes, especially of histologic staining. It is possible 



*' R. Keller, Elektrohistologische Untersuchungen an Pflanzen und Tieren. 

 Prag 1920. 



