CHAP. XL] ELECTRICITY AND MAGNETISM. 379 



CHAPTER XL 



ELECTRO-CHEMISTRY. 

 LESSON XXXVIII. Electrolysis and Electrometallurgy . 



412. In Lessons XIV. and XVIII. it was explained 

 that a definite amount of chemical action in a cell 

 evolves a current and transfers a certain quantity of 

 electricity through the circuit, and that, conversely, a 

 definite quantity of electricity, in passing through an 

 electrolytic cell, will perform there a definite amount of 

 chemical work. The relation between the current and 

 the chemical work performed by it is laid down in the 

 following paragraphs. 



413. Electromotive - force of Polarisation. 

 Whenever an electrolyte is decomposed by a current, 

 the resolved ions have a tendency to reunite, that 

 tendency being commonly termed " chemical affinity." 

 Thus, when zinc sulphate (Zn SO 4 ) is split up into Zn 

 and SO 4 the zinc tends to dissolve again into the solution 

 by reason of its " affinity " for oxygen and for sulphuric 

 acid. But zinc dissolving into sulphuric acid sets up an 

 electromotive-force of definite amount ; and to tear the 

 zinc away from the sulphuric acid requires an electro- 

 motive-force at least as great as this, and in an opposite 

 direction to it. So, again, when acidulated water is 

 decomposed in a voltameter, the separated hydrogen 



