THE ELECTRIC CURRENT. I/ 



If the zinc is connected to a carbon or copper electrode, however, or if a piece of car. 

 bon or copper touches the zinc plate in the solution, chemical action begins at once, 

 current flows through the electrolyte from the zinc to carbon or copper and back 

 through the metallic connection to the zinc, the sulphuric acid is decomposed, hydro- 

 gen is liberated at the carbon or copper electrode, and SO 4 is liberated at the zinc 

 electrode where it combines with the zinc forming zinc sulphate. When a plate of 

 impure zinc is immersed in dilute sulphuric acid, the insoluble impurities are left in 

 the form of fine particles clinging to the surface of the zinc after the zinc is partly dis- 

 solved, and these fine particles play the part of carbon or copper cathodes, current flows 

 through the acid from the zinc to each particle and back to the zinc through the point 

 of attachment of the particle with the zinc plate, as indicated in Fig. loa, the acid is 

 decomposed, hydrogen is liberated at the surface of each par- 

 ticle, and SO 4 is liberated at the surface of the zinc plate 

 where it combines with the zinc forming zinc sulphate. The 



rapid dissolving of impure zinc in sulphuric acid is no \ zinc 



doubt due to the flow of electric currents through the min- 

 ute "local circuits" as here described, and this rapid dis- 

 solving of impure zinc is therefore called local action. 



The covering of the zinc plate with a thin layer of me- 

 tallic mercury tends to produce a clean metallic surface 

 which is free from adhering particles of the impurity which 

 is left as the zinc wastes away, and the above described ac- p lg 10a 



tion does not take place. It is probable that in some cases 



chemical action (local action) takes place irrespective of the flow of electric currents 

 in local circuits as above described. This seems to be the case, for example, in the 

 chromic acid cell, for, as a matter of fact, more than three fourths of the zinc in such 

 a cell is consumed independently of voltaic action, even when the zinc is thoroughly 

 amalgamated so as to present a clean bright surface, but in the chromic acid cell the 

 local action is very much less when the zinc is amalgamated than it is when the zinc 

 is not amalgamated. 



An essential feature of voltaic action is that it is reversed if a 

 current is forced backwards through a voltaic cell by an outside 

 agent, provided that no material that has played a part in the 

 previous voltaic action has been allowed to escape from the 

 cell. Thus in the operation of the simple voltaic cell consisting 

 of a zinc anode and a carbon cathode in dilute sulphuric acid, the 

 H 2 SO 4 is decomposed, ZnSO 4 is formed at the anode, and hydro- 

 gen is liberated at the cathode. If the current is reversed so that 

 the carbon plate becomes the anode, and the zinc plate the 

 cathode, then the ZnSO 4 , previously formed, will be decomposed, 

 metallic zinc will be deposited upon the zinc cathode, and SO 4 

 will be liberated at the carbon anode where it will combine with 

 3 



