xii CONSTRUCTION OF SIMPLE VOLTAIC CELLS 189 



explain the chemical changes occurring in the simple cell already 

 described. The facts we have learnt are first, that the zinc gradu- 

 ally dissolves in the acid, and second, bubbles of hydrogen con- 

 tinually collect on the copper plate. Since we have proved by 

 experiment that dilute sulphuric acid has no action on copper, it is 

 evident that though the bubbles of hydrogen collect on the copper 

 plate they were not turned out of the sulphuric acid by the copper. 

 As we shall learn in the chemical section of this book, the 

 chemical change which takes place when ordinary commercial zinc 

 is placed in sulphuric acid can be represented by an equation : 



When SOIPHOIHC ACID ZXNC formed 



and HYDROGEN & 

 or more shortly by chemical symbols, 



The hydrogen is, then, really first liberated near the piece of zinc, 

 and it is supposed that at the moment of its liberation it breaks 

 up the adjoining molecule of sulphuric acid, taking the place of 

 the hydrogen therein, and turning out that which was already com- 

 bined with the other elements in the molecule of sulphuric acid. 

 This can be shown a little more clearly, perhaps, by using 

 the symbol H' for the hydrogen first turned out by the zinc, and 

 H" for that in the neighbouring molecule of sulphuric acid of 

 which we have spoken. Then the interchange of hydrogen in this 

 molecule of sulphuric acid can be expressed by an equation : 



This interchange of the hydrogen goes on right through the 

 liquid until the copper plate is reached. We have the same 

 reaction going on an almost infinite number of times : 



H"' 2 SO 4 + H" 2 = H" 2 SO 4 + H'" 2 



until finally there is no longer a molecule of sulphuric acid to be 

 decomposed, and the free hydrogen is deposited on the copper 

 plate. It is customary to represent the state of affairs in the cell 

 graphically, and since it is very important that a proper concep- 

 tion of the continuous decomposition of sulphuric acid molecules 

 should be formed, we also give this method, as it may help 

 some readers better than the previous plan. In the illustration 

 (Fig. 97) the second row of oval figures, stretching between the 

 zinc and copper plates, A and B respectively, is to represent the 



