512 



SCIENCE. 



[N. S. Vol. IV. No. 93. 



centration in solutions of electrolytes about 

 the electrodes when a current passes- 

 This phenomenon had been noticed and 

 studied to some extent by Daniell and 

 others, without, however, having been made 

 use of in explaining the nature of electro- 

 lytic action. Hittorf's studies and con- 

 clusion bring us into the very midst of the 

 modern views of electrolysis. Taking a 

 simple case, let two electrodes of copper be 

 placed vertically over one another in a solu- 

 tion of copper sulphate, pass a current 

 through the cell making the lower plate the 

 anode; no very noticeable change occurs 

 other than that copper is dissolved at the 

 anode and deposited at the kathode; if after 

 the current has passed for a short time it is 

 interrupted and the electrode short cir- 

 cuited through a galvanometer, a current 

 will for a short time flow in the cell from 

 kathode to anode, that is, in a direction op- 

 posite to the one which has passed through 

 in electrolyzing. The counter-electromotive 

 force in this case can not result from polar- 

 ization at the electrodes, for no change has 

 been brought about at their surfaces, such 

 as a gas deposit in the case of decomposi- 

 tion in acidulated water with platinum 

 electrodes. If the electrolyzing current be 

 continued for some time it will be seen that 

 the solution about the anode has become 

 more concentrated and more dilute about 

 the kathode; the total quantity of copper salt 

 in the solution having, however, remained 

 the same, the counter- electromotive force 

 above referred to is due to this change in 

 concentration. 



Hittorf, from 1853 to 1859, examined in a 

 most careful manner the behavior of many 

 electrolytes, and by a series of analyses of 

 the solutions determined this change of 

 concentration due to the passage of the 

 current. His patient labor has only within 

 the last few years received proper recogni- 

 tion. 



All theories which so far had survived 



the test, conceived that the electricity was 

 conveyed by a migration of particles, called 

 ions by Faraday; whether these particles 

 received their changes by contact with the 

 electrodes or contained definite inherent 

 quantities of electricity, the charges being 

 the same for all ions, need not be considered 

 at this time. 



The first step toward the decisive estab- 

 lishment of the fact of the migration of the 

 ions towards the plate would be an experi- 

 mental measurement of the rate of migra- 

 tion ; this was accomplished by Hittorf and 

 led to the enunciation of these laws: 



1. The change in concentration due to 

 current, is determined by the motion which 

 the ions have in the unchanged solution. 



2. The unlike ions must have different 

 velocities to produce such change in con- 

 centration. 



3. The numbers which express ionic 

 velocities mean the relative distance 

 through which the ions move between the 

 salt molecules, or express their relative 

 velocities in reference to the solution, the 

 change in concentration being a function of 

 the relative ionic velocities. Hittorf's 

 analyses enabled him to give their numer- 

 ical values. A great many such have been 

 made by him, Nernst, Loeb and others; 

 these results show that in dilute solutions 

 the relative velocities of the ions are inde- 

 pendent of the difference in potential be- 

 tween the electrodes (if the current be 

 steady), and that they are only slightly af- 

 fected by temperature. 



Hittorf points out that a knowledge of 

 the specific resistance of electrolytes should 

 give valuable information in reference to 

 the nature of electrolytic action. Hors- 

 ford, Wiedemann and Beez made such meas- 

 urements ; their methods were, however, 

 imperfect; it remained for P. Kohlrausch 

 to. devise a method, using an alternating 

 current, by means of which accurate results 

 were obtained. Kohlrausch 's work shows 



