60 



ELECTROLYSIS. 



to be the prototype of the two-liquid cell. The latter is, 

 however, more constant. 



The object of the two compartments is to prevent the 

 polarisation by interposing, between the two electrodes, a liquid 

 which does not attack the positive metal, while at the same 

 time it supplies sufficient quantities of oxygen for consuming 

 the hydrogen as fast as the latter is becoming deposited on the 

 negative metal. 



DANIELL CELL. The Daniell cell, as most generally in 

 use in France, consists of an external vessel made of earthen- 

 ware, a hollow cylinder of zinc, an internal porous china 

 vessel, and a central strip of copper. In the internal porous 

 vessel is a saturated solution of sulphate of copper, and in the 

 external vessel dilute sulphuric acid. The copper electrode 

 dips into the sulphate, and the zinc one into the acidulated 

 water. 



Fig. 1 is an illustration of the Daniell cell as it is in 

 actual use. The zinc is dissolved by oxidisation, and forms 

 sulphate of zinc ; the hydrogen, which 

 results from this reaction, passing through 

 the porous vessel, substitutes itself in the 

 sulphate for an equal quantity of copper, 

 the latter being deposited on the negative 

 electrode. 



After a certain time of working, all 

 the sulphuric acid of the external vessel 

 is converted into sulphate of zinc ; the 

 action, nevertheless, continues without 

 any change in the electromotive force. 

 The chemical reactions are then reduced 

 .. to the substitution of the zinc for the 



copper in the sulphate of copper, and to 

 the progressive conversion of the sulphate of copper into sul- 

 phate of zinc. It is even unnecessary to use dilute sulphuric acid 

 when charging the cell, pure water being sufficient ; the action is 

 weaker at starting, but the radical S0 4 rapidly gets through 

 the partition, and is converted into sulphate of zinc by the 

 action of the zinc. 



