XTI CONSTRUCTION OF SIMPLE VOLTAIC CELLS 191 



Prevention of Polarisation by Chemical Means 1. Daniell's 

 Cell. _ The deposition of copper instead of hydrogen. In most of 

 the cells in which polarisation is prevented by chemical means 

 there are really two cells, the one placed inside the other. The 

 inner one is made of some porous kind of earthenware which 

 permits a slow passage through it of the liquids on either side of 

 it. In Daniell's cell the outer vessel is of copper and serves as 

 the copper plate. The outer vessel contains a solution of copper 

 sulphate (blue vitriol), the strength of which is maintained by 

 placing some crystals of the same substance on a tray, which 

 extends round the top of the inside of the copper vessel (Fig. 98). 

 The inner porous pot contains dilute 

 sulphuric acid into which dips a rod of 

 amalgamated zinc. 



The chemical action, on which the 

 production of the electric current de- 

 pends, begins in the neighbourhood of 

 the zinc rod, and is of precisely the 

 same nature as that which has been 

 seen to occur in a simple voltaic cell. 

 There is a simultaneous production of 

 zinc sulphate and free hydrogen. This 

 hydrogen similarly decomposing the 

 neighbouring molecule of sulphuric 

 acid results in the liberation of more hydrogen, this action being 

 reproduced throughout the row of sulphuric acid molecules 

 between the zinc rod and the porous vessel. The free hydrogen 

 which is there formed, and which would in the simple voltaic 

 cell be deposited on the copper plate, passes through the pores 

 of the earthenware partition and comes into contact with the 

 copper sulphate solution. A chemical reaction ensues. The 

 hydrogen, being just liberated from combination, is in a parti- 

 cularly active state, known as the nascent condition, and is under 

 these circumstances able to effect the decomposition of the copper 

 sulphate. We can represent the change which follows thus : 



H 



FIG. 98. Daniell's Cell. 



The same reaction is more shortly expressed by a chemical 

 equation : 



