192 ELEMENTARY GENERAL SCIENCE CHAP. 



The free copper must now be supposed to act upon the neigh- 

 bouring molecule of copper sulphate decomposing it, forming 

 copper sulphate, which contains the same proportions of the 

 constituent elements of course, but not the identical atoms. 

 Just as in the case of the sulphuric acid in the inner cell, so here 

 in the outer, there is a series of decompositions and recomposi- 

 tions, with the final result that there is eventually a deposit of 

 free copper formed upon the inside of the copper vessel, but no 

 hydrogen bubbles. 



The cell continues to develop an electric current at the expense 

 of the zinc, which is gradually dissolved. Throughout the passage 

 of the electric current round the wire joining the zinc rod and the 

 copper vessel, there is a continuous deposition of metallic copper 

 on the inside of the copper vessel, which consequently gets heavier 

 and heavier. 



This form of cell does not give a very strong current, but the 

 great advantage about it is that the strength of the current is 

 very uniform. It is largely used by the Post Office authorities 

 for telegraphic purposes. 



2. Bunsen's and Grove's Cells. The hydrogen is got rid of 

 by oxidation. The only difference between these two kinds of 

 voltaic cells is that, whereas, in the former a piece of gas carbon 

 replaces the copper plate, in the latter there is a plate of plati- 

 num. Owing to the cheapness of gas carbon Bunsen's cell is the 

 most commonly used. 



In Bunsen's cell there are two earthenware vessels, the inner 

 smaller one is porous and is filled with strong nitric acid, into 

 which the piece of gas coke C dips. The 

 outer vessel contains dilute sulphuric 

 acid, and in it is placed the zinc plate Z, 

 which is usually made cylindrical in 

 shape. The arrangement of the parts 

 is easily understood by a reference to 

 Fig 99. 



The chemical action begins at the 

 zinc plate and the result of it is that, 

 as in Daniell's cell, free hydrogen passes 

 through the porous vessel and comes 

 Fia. 99. Bunsen's Cell. into contact with the strong nitric acid 

 contained in it. Nitric acid is remark- 

 able for the large quantity of oxygen it contains and for the ease 

 with which it parts with it. It is hence known as an oxidising 



