STORAGE BATTERIES. 239 



In many cases the cathion and anion are not actually liberated 

 at the electrodes, because of what are called secondary reactions. 

 Thus in the electrolysis of an aqueous solution of sodium chloride 

 (NaCl), the cathion Na, when it is liberated at the cathode, im- 

 mediately reacts upon the water forming NaOH and free hydro- 

 gen ; in the electrolysis of copper sulphate (CuSOJ between 

 copper electrodes, the anion SO 4 combines with the copper of 

 the anode forming fresh CuSO 4 which goes into solution, or 

 which is deposited in crystals on the anode if the solution is 

 saturated ; in the electrolysis of H 2 SO 4 between inert electrodes 

 the hydrogen is liberated at the cathode as a gas, and the anion 

 SO 4 reacts on the water according to the formula SO 4 -f H 2 O 

 = H 2 SO 4 -f O, and the free oxygen escapes as a gas. The 

 reason for taking the unfamiliar substance, hydrobromic acid, in 

 the above example is that in the electrolysis of hydrobromic acid 

 there are no secondary reactions at the electrodes. 



102. The voltaic cell.* The chemical action that is caused by 

 the flow of current through an electrolytic cell is confined wholly 

 to the immediate neighborhood of the electrodes, and this chem- 

 ical action is usually forced, that is, work has to be done to bring 

 it about, or, in other words, an outside electromotive force is re- 

 quired to push the current through the cell. 



When, however, secondary chemical actions take place at one 

 or both electrodes, it frequently happens that the total chemical 

 action that is brought about by the flow of current through an 

 electrolytic cell is a source of energy. In such a case the elec- 

 trolytic cell itself can maintain a current through the electrolyte 

 from electrode to electrode and through an outside circuit of wire 

 which connects the electrodes. Such an electrolytic cell is called 

 a voltaic cell or primary battery. 



Examples. When a strip of clean zinc and a strip of copper or 

 carbon are dipped into dilute sulphuric acid, no chemical action 

 takes place. When the plates are connected together by a wire, 



* A number of voltaic cells connected together constitute a voltaic battery. The 

 word battery is, however, frequently applied to a single cell. 



