PRACTICAL PHYSIOLOGY 



plate in a saturated solution of copper sulphate 

 and introduced a porous pot to separate this from 

 the dilute sulphuric acid in which the zinc is im- 

 mersed. The zinc then dissolves in the acid, dis- 

 placing hydrogen ; the hydrogen in its turn dis- 

 places copper from the copper sulphate, and the 



Zinc 



Carbon. 



Manganese dioxide 

 in porous pot. 



Ammonium chloride 

 solution. 



FIG. 4. LECLANCHE CELL. 



displaced copper is deposited on the copper plate, 

 so that no bubbles of hydrogen are formed upon 

 the metal, and if the copper sulphate solution 

 is kept saturated, the E.M.F. of the cell remains 

 constant. Commercial zinc, which is never pure, 

 must always be " amalgamated " by rubbing its 

 surface with mercury after it has been cleaned by 

 dipping into acid. 



