Electrolysis of certain Metallic Chlorides. 427 



A small bell-jar which terminated in a corked orifice was in- 

 verted and partly filled with mercury, over which was poured a 

 solution of mercuric chloride ; a wire passed from the mercury 

 through the cork to a plate of platinum which hung in the solu- 

 tion, without, however, touching the mercury. On pouring the 

 mercuric chloride upon the mercury, the metallic surface was at 

 once dimmed by a film of mercurous chloride, which increased in 

 quantity ; and in the course of an hour or two the insoluble chlo- 

 ride appeared also on the platinum plate, and in twenty-four 

 hours it was sufficiently thick to permit of its removal. On repeat- 

 ing this experiment with gold instead of platinum, the same mer- 

 curous chloride was deposited ; but at the same time the gold 

 plate was amalgamated, showing that the reduction of the mer- 

 curic chloride had not stopped at the first stage, but had actu- 

 ally proceeded to the separation of the metal itself. A similar 

 gold plate immersed in the same solution of corrosive sublimate, 

 but not in connexion with the mercury, showed no trace of de- 

 posit or amalgamation ; and it was ascertained that gold alone has 

 no power of decomposing moistened mercurous chloride. 



An experiment was made with a current ab extra. A cell of 

 Grove's was found to decompose mercuric chloride with the for- 

 mation of the mercurous compound at the negative platinum 

 electrode, while chlorine was given off at the positive one. This 

 is in unison with what was found in the case of the copper salts. 



That this action does not depend on the insolubility of the 

 -ous chlorides was proved by the behaviour of the iron salts. 

 Ordinary metallic iron is capable of reducing the ferric to the fer- 

 rous salt at the common temperature ; thus : — » 



2FeCl 3 + Fe=3FeCL, 



Platinum does not effect such a reduction ; but when the iron is 

 connected with platinum the change takes place more rapidly, 

 and the reduced salt forms also on the negative metal. This, of 

 course, does not render itself evident by any deposit, nor by any 

 appreciable change of colour at first; but if the platinum plate 

 be lifted out of the solution, and the liquid clinging to it be 

 allowed to drain on to a paper moistened with some ferridcyanide 

 of potassium, TurnbulPs blue is the result. Or if a few drops 

 of the ferridcyanide be mixed with the solution of the ferric chlo- 

 ride, on the junction of the iron and platinum the blue colour 

 makes its appearance against each metal. Of course care was 

 taken that the original salt contained no acid. The solution 

 employed was one of 3*5 per cent, in strength. 



If plates of magnesium and platinum be immersed in ferric 

 chloride, metallic iron quickly makes its appearance on the pla- 

 tinum plate. 



