at the Electrodes in a Solution. 



71 



After this is the case, diffusion can no longer bring sufficient 

 copper to the electrode to cany the current, for the formula 

 shows that if the current were still conducted in the same 

 manner as it was before, the concentration of the copper 

 would become negative. Tt might therefore be supposed 

 that it would be impossible to keep the current at its 

 former strength. This is, however, found not to be the 

 case. 



In 1844 Smee* described an experiment, in which he 

 electrolysed a solution of pure copper sulphate in a tall vessel, 

 in the upper part of which he placed the cathode. His 

 description of the varying appearance presented by the 

 deposition of the copper as the concentration of his solution 

 went down to zero runs: — " On the action of the galvanic force 

 bright reguline copper first appeared at the negative pole, 

 this was followed by a brittle, this by a sandy, this by a 

 spongy deposit, this by black powder, and finally hydrogen 

 was evolved/'' I have in general been able to verify Smee's 

 observations as well as the fact, mentioned by him, that 

 copper hydrate is formed at the electrode after the black 

 powder has begun to appear. I have, however, in no case — 

 neither when employing apparatus o, nor when a form of 

 apparatus was employed in which the solution was not con- 

 tained in a closed vessel — been able to observe bubbles of 

 hydrogen. . It is therefore probable that Smee's copper- 

 sulphate solution was more acid than mine. In my experi- 

 ments the black spongy deposit always seemed to grow into 

 the solution; in single cases it became intermingled with 

 bright branches of metallic copper. The black deposit is 

 evidently not pure copper, for it disappears when left for 

 some time in dilute sulphuric acid. It is probably a copper 

 hydride f. 



The behaviour of the current while electrolysing a solution 

 of pure copper sulphate w r as quite similar to that when an 

 acid solution was examined. The black deposit always ap- 

 peared after the large drop had taken place. The completion 

 of the drop in the current was taken as the time when the 

 concentration of the copper had gone down to zero. From a 

 knowledge of this time and of the current-density employed, 

 it is possible to calculate the diffusion-coefficient of copper 

 sulphate bv means of the expression given at the end of 

 Table I. this has been done in Table VI. (p. 7*2). 



* Phil. Mag. xxv. p. 437. 



f See Pogg. Ann. lxxv, }>. 350. 



