ELEMENTS OF ELECTRICITY AND MAGNETISM. 



as saw-dust, the containing vessel is made of zinc and serves as 

 the zinc electrode, and the cell is hermetically sealed so as to pre- 

 vent evaporation. 



The ordinary gravity cell, which is shown in Fig. 6, consists 

 of a copper electrode in the bottom of a jar surrounded by a solu- 

 tion of copper sulphate, and a zinc electrode in the top of the jar 

 surrounded by a solution of zinc sulphate. The light zinc sul- 

 phate solution floats on the heavy copper sulphate solution. 

 When this cell delivers current, SO 4 is liberated at the zinc 

 electrode where it combines with the zinc forming additional zinc 



18 OP 

 6UUPHAT5 , 



Fig. 6. 



Fig. 7. 



sulphate, and metallic copper is deposited upon the copper elec- 

 trode at the bottom of the cell. When this cell is in use, the 

 copper sulphate must be replenished occasionally by dropping 

 fresh crystals of the salt into the cell, and a portion of the zinc 

 sulphate solution must be occasionally drawn off and replaced by 

 water. 



The chromic acid cell consists of a plate of amalgamated zinc 

 and a plate of carbon dipping into a solution of a mixture of 

 chromic acid (H 2 Cr 2 O 7 ) and sulphuric acid (H 2 SO 4 ). When 

 this cell delivers current, the flow of the current through the cell 

 decomposes the H 2 SO 4 . The acid radical SO 4 is liberated at 



