132 ELEMENTARY LESSONS ON [CHAP. in. 



Oxygen; or, in symbols, H 2 SO 4 . The chemical reaction 

 by which the zinc enters into combination with the 

 radical of the acid, turning out the hydrogen, is expressed 

 in the following equation : 



Zn + H 2 SO 4 Zn SO 4 + H 2 



Zinc and Sulphuric Acid produce Sulphate of Zinc and Hydrogen. 



The sulphate of zmc produced in this reaction remains 

 in solution in the liquid. 



Now, when a plate of pure zinc and a plate of some 

 less-easily oxidisable metal copper or platinum, or, best 

 of all, carbon (the hard carbon from the gas retorts) 

 are put side by side into the cell containing acid, no 

 appreciable chemical action takes place until the circuit 

 is completed by joining the two plates with a wire, or by 

 making them touch one another. Directly the circuit is 

 completed a current flows and the chemical actions 

 begin, the zinc dissolving in the acid, and the acid giving 

 up its hydrogen in streams of bubbles. But it will be 

 noticed that these bubbles of hydrogen are evolved not 

 at the zinc plate, nor yet throughout the liquid, but at the 

 surface of the copper plate (or the carbon plate if carbon 

 is employed). This apparent transfer of the hydrogen 

 gas through the liquid from the surface of the zinc plate 

 to the surface of the copper plate where it appears is 

 very remarkable. The ingenious theory framed by 

 Grotthuss to account for it, is explained in Lesson 

 XXXVIII. on Electro-Chemistry. 



These chemical actions go on as long as the current 

 passes. The quantity of zinc used up in each cell is 

 proportional to the amount of electricity which flows 

 round the circuit while the battery is at work ; or, in 

 other words, is proportional to the strength of the 

 current. The quantity of hydrogen gas evolved is also 

 proportional to the amount of zinc consumed, and also 

 to the strength of the current. After the acid has thus 

 dissolved zinc in it, it will no longer act as a corrosive 



