- CHAPTER VIII. 

 STORAGE BATTERIES. 



101. Electrolysis. When an electric current passes through 

 a conducting liquid which is not a chemical element the liquid is 

 decomposed. Thus melted salts, and acids and salts in solution, 

 are decomposed by the electric current. This chemical decom- 

 position is called electrolysis, and the liquid in which electrolysis 

 takes place is called an electrolyte. 



Electrolytes generally have very high specific resistances as 

 compared with the metals, so that electrolysis is usually carried 

 out in a vessel provided with two flat plates of metal or carbon 

 which serve to lead the current into and out of the electrolyte. 

 Such an arrangement is called an electrolytic cell and the plates of 

 metal or carbon are called the electrodes. The electrode at which 

 the current enters the electrolyte is called the anode, and the 

 electrode at which the current leaves the electrolyte is called the 

 cathode. 



Consider a solution of hydrobromic acid (HBr). When an 

 electric current is passed through this solution, hydrogen (H) is 

 liberated at the cathode and bromine (Br) is liberated at the anode. 

 In general the molecule of any dissolved salt or acid is separated 

 into two parts by electrolysis ; one part is liberated at the cathode 

 and is called the cathion, and the other part is liberated at the 

 anode and is called the anion. Thus hydrogen (H) is the cathion 

 and bromine (Br) is the anion of hydrobromic acid. In all me- 

 tallic salts the metal constitutes the cathion, and the acid radical 

 or halogen constitutes the anion. In acids the hydrogen consti- 

 tutes the cathion, and the acid radical or halogen constitutes the 

 anion. Thus the cathion of copper sulphate (CuSO 4 ) is copper 

 (Cu), and the anion is the acid radical (SO 4 ). 



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