162 Dr. Jacobi on various subjects of 



a cloison where the engraved plate is used in the place of the 

 ordinary copper plate, being plunged in the solution of sulphate 

 of copper. I have found it necessary that a galvanometer 

 with short wires should always make part of the circuit, so 

 that one may judge of the force of the current and direct the 

 action; the latter being effected by separating the electro- 

 motive plates more or less from each other or modifying the 

 length of the conjunctive wire, or finally, diminishing more 

 or less the conducting power of the liquid on the zinc side ; 

 but for the success of the operation it is of great importance 

 that the solution of copper should be always perfecdy satu- 

 rated. The action should not be too rapi-d : from 50 to 60 

 grains of copper should be reduced on each square inch in 

 24 hours. The accompanying plates have been formed, one in 

 two days, the other in one day only, and that is the reason why 

 their state of aggregation is not so solid and compact as that of 

 the small piece, No. 4, which has been reduced more slowly. 



It is to be understood that we may reduce the sulphate of 

 copper by making the current of a single voltaic pair pass 

 through the solution by copper electrodes; as the anode is 

 oxidized the cathode becomes covered with reduced copper, 

 and the supply of concentrated solution may then be dis- 

 pensed with. According to theory one might expect that ex- 

 actly the same quantity of copper oxidized on one side would 

 be reduced on the other, but I have always found a difference 

 more or less great, so that the anode loses more than the cath- 

 ode gains. The difference appears to be nearly constant, 

 for it does not augment after a certain time, if the experiment be 

 prolonged. A thoroughly concentrated solution of sulphate 

 of copper is not decomposable by electrodes of the same metal, 

 even on employing a battery of three or four pairs of plates. 

 The needle is certainly strongly affected as soon as the cir- 

 cuit is completed, but the deviation visibly diminishes and 

 very soon returns almost to zero. If the solution be diluted 

 with water to which a few drops of sulphuric acid have been 

 added, the current becomes very strong and constant, the 

 decomposition goes on very regularly, and the engraved cath- 

 ode becomes covered with copper of a fine pink red co- 

 lour. If we replace the solution of sulphate of copper by 

 pure water acidulated with sulphuric acid, there is a strong 

 decomposition of water even on employing a single voltaic 

 couple. The anode is oxidized, and hydrogen is disengaged 

 at the cathode. At the commencement the reduction of 

 copper does not take place ; it begins as soon as the liquid 

 acquires a blue colour, but its state of aggregation is al- 

 ways incoherent. I have continued this experiment for three 



