ON ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 349 



constant for the same current when prolonged. But at an intensity 8-6 a new 

 phenomenon is produced : polarisation increases with time, first ver}' slowly, then 

 more and more rapidly, going from -04 to '65 volt ; at the same time one notices 

 that the metallic and brilliant deposit of pure copper which one had hitherto 

 obtained is displaced by a ruddy and non-adhesive deposit. In proportion as it is 

 produced the polarisation increases, and the deposit overspreads the electrode with 

 increasing rapidity. 



' Finally, augmenting the current still more, the deposit passes gradually from 

 red to black, while polarisation increases in a continuous manner, and for a sufficient 

 current-intensity the deposit acquires anew a certain adherence. It is then dark- 

 gTey, very rich in zinc, and recalls by its aspect deposits of zinc obtained from- 

 impure commercial sulphate of zinc. 



' As for the conductmti/ of the liquid it reniaiivi constant all the time, in spite of 

 the variety of electrolytic actions, to which a study of polarisation and aspect of 

 deposit bear witness. . . . The same kind of thing happens with other propor- 

 tions of CuSO^ and ZnSO,. . . . 



' One may remark further that the specific resistance of the liquid passes through 

 a minimufti for a certain composition of the mixture ; it is then inferior to the 

 resistance of even a saturated solution of one of the two salts, and a fortiori to that 

 of the same salt diluted down to the strength in which it occui-s in the liquor. So 

 the molecules of tioo mixed salts take part in the transport of electricity, even when 

 only one of the two metals is deposited on the cathode.^ 



All the variations of polarisation in the above case are then simply and naturally 

 explained by the fact of exhaustion, in the liquid near cathode, of the salt of the 

 metal being deposited, except in so far as diffusion replenishes it. With strong 

 currents it is therefore plainly necessary for zinc to be deposited as well as copper, 

 and it is equally obvious that this zinc will tend to clear itself off again by local action. 



The author then goes on to observe that very similar complications occur even 

 when only one salt is intended to be present. Thus pure CuSO^ almost always 

 contains a trace of acid, and accordingly, in its solution, hydrogen plays much the 

 same part as zinc has done in the above described experiment. For feeble in- 

 tensities copper alone is deposited, but for stronger currents the deposit is red and 

 contains some oxide [?]. Hydrogenised copper forms with copper, in fact, local 

 couples in which copper is the attacked element. Evolution of heat by local 

 action has been proved by the use of thermometer electrodes. 



Even if OuSO^ contained no acid to start with, it would soon get some by 

 electrolysis ; > for the solution of anode is never exactly equal to deposit on cathode. 



In all these cases one may notice that electrolytic reactions which go on for the 

 most feeble currents absorb ahoays less heat than those lohich occur with stronger 

 currents. This extension to mixtures of the beautiful law announced by Berthelot 

 for the case of electrolysis of a single salt is confirmed by a study of particular 

 cases. For instance, the following table sums up the author's observations on a 

 mixture of {^ by volume of a solution of Na.^SO^, and i of a solution of CuSO^, 

 both pure and saturated when cold. The polarisation of anode is so small as to be 

 negligible; the polarisation of cathode is given for various intensities of current 

 in ten- thousandths of an ampere per square centimetre. 



Current Intensity Polarisation of Cathode in Volts 



7 ■ •042') 



10 -044 



1.') -04.5 



19 -051 



31 -068 



36 -100 



41 -162 



44-7 -298 



45-4 • -859 1 „ A -t- 



1 .o^„ > Brown deposit. 



150 1-585 Abundant evolution of hydrogen. 



' Unless one artificially keeps it neutral : see D'Almeida, A mmles de CJiimie^ 

 3e serie, t. ii. p. 257. 



" Brilliant deposit of copper. 



