ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 257 



cm. is obtained. Perrin first of all found that only liquids of pro- 

 nounced ionizing capacity showed strong electroendosmosis. Liquids 

 with small dielectric constants which are poor conductors and which 

 permit but little dissociation of electrolytes dissolved in them, dis- 

 played very weak or no endosmosis. Thus chloroform, petroleum 

 ether, benzene, oil of turpentine, carbon bisulfide showed no en- 

 dosmosis at all, in contrast to nitrobenzene, acetone, ethyl and 

 methyl alcohols, and above all to water which endosmotically are 

 very active. 



The electrolytes dissolved in the water have a great influence upon 

 the extent and direction of the endosmosis. 

 Of particularly great effect are the acids and 

 alkali bases, hence the H- and the OH-ions. 

 Perrin found a whole series of powdered sub- 

 stances with which the direction of the endos- 

 mosis in faintly acidified water was the reverse 

 of that in weakly alkaline water. In the acid 

 solution the water always migrated towards the 

 anode, and towards the cathode in the alkaline 

 solution, which means that the powdered sub- 

 stance was positively charged towards the acid 

 and negatively charged towards the alkahne 

 solution. Such substances were charcoal, sul- 

 fur, carborundum, naphthalene, salol, CrCls, 

 AgCl, BaS04, ZnO, CuO, gelatin and others. 

 Table 38 shows the results for a few of the sub- 

 stances in a diaphragm of 1.4 cm. diameter, 

 in cubic millimeters of water per minute. 



It is seen from this table that, as a rule, the substances studied 

 have a positive charge in an acid medium and a negative charge in a 

 basic medium. But the reversal of the sign of the charge is certainly 

 not at true neutrality. Indeed Perrin beheved at the time that the 

 cause for this lay in secondary disturbances in the experimental con- 

 ditions, such as the gradual solution of the powdered substance and 

 the change of reaction resulting therefrom, or the presence of traces of 

 polyvalent ions which, as we shall soon see, have a profound effect 

 upon this process. Perrin deduced the following law: 



Every non-metallic substance {in the absence of polyvalent ions) is 

 positively charged in an acid solution and negatively in an alkaline 

 solution. 



Fig. 29 



