EXPERIMENTS WITH THE VOLTAIC CELL 307 



copper and a strip of zinc are placed in a glass containing 

 dilute sulphuric acid, a solution composed of oxygen, hydro- 

 gen, sulphur, and water. As soon as the plates are immersed 

 in the acid solution, minute bubbles of gas rise from the zinc 

 strip and it begins to waste away slowly. The solution 

 gradually dissolves the zinc and at the same time gives up 

 some of the hydrogen which it contains ; but it has little or 

 no effect on the copper, since there is no visible change in 

 the copper strip. 



If, now, the strips are connected by means of metal wires, 

 the zinc wastes away rapidly, numerous bubbles of hydrogen 

 pass over to the copper strip and collect on it, and a current 

 of electricity flows through the connecting wires. Evidently, 

 the source of the current is the chemical action between the 

 zinc and the liquid. 



Mere inspection of the connecting wire will not enable 

 us to detect that a current is flowing, but there are vari- 

 ous ways in which the current makes itself evident. If the 

 ends of the wires attached to the strips are brought in con- 

 tact, a faint spark passes, and if the ends are placed on the 

 tongue, a twinge is felt. 



282. Experiments which grew out of the Voltaic Cell. 

 Since chemical action on the zinc is the source of the cur- 

 rent, it would seem reasonable to expect a current if the cell 

 consisted of two zinc plates instead of one zinc plate and 

 one copper plate. But when the copper strip is replaced by 

 a zinc strip so that the cell consists of two similar plates, no 

 current flows between them. In this case, chemical action is 

 expended in heat rather than in the production of electricity 

 and the liquid becomes hot. But if carbon and zinc are 

 used, a current is again produced, the zinc dissolving away 

 as before, and bubbles collecting on the carbon plate. By 

 experiment it has been found that many different metals may 



