CHAP, in.] ELECTRICITY AND MAGNETISM. 



139 



reduction of the liquid. Complete depolarisation is only 

 obtained by two-fluid cells. 



TWO-FLUID BATTERIES. 



17O. Daniell's Battery. Each cell or " element " 

 of Daniell's Battery consists of an inner and an outer 

 cell, divided by a porous partition 

 to keep the separate liquids in 

 the two cells from mixing. The 

 outer cell (Fig. 73) is usually of /" 

 copper, and serves also as a ^ 

 copper plate. Within it is placed 

 a cylindrical cell of unglazed 

 porous porcelain (a cell of parch- 

 ment, or even of brown paper, 

 will answer), and in this is a 

 rod of amalgamated zinc for the 

 negative pole. The liquid in 

 the inner cell is dilute sulphuric 

 acid ; that in the outer cell is a saturated solution of 

 sulphate of copper (" blue vitriol "), some spare crystals 

 of the same substance being contained in a perforated 

 shelf at the top of the cell, in order that they may 

 dissolve and replace that which is used up while the 

 battery is in action. 



When the circuit is closed the zinc dissolves in the 

 dilute acid, forming sulphate of zinc, and liberating hydro- 

 gen gas ; but this gas does not appear in bubbles on the 

 surface of the copper cell, for, since the inner cell is 

 porous, the molecular actions (by which the freed atoms 

 of hydrogen are, as explained by Fig. 155, handed on 

 through the acid) traverse the pores of the inner cell, 

 and there, in the solution of sulphate of copper, the 

 hydrogen atoms are exchanged for copper atoms, the 

 result being that pure copper, and not hydrogen gas, is 

 deposited on the outer coppe-r plate. Chemically these 



