190 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



condition of the molecules of the acid before the plates are 

 joined by a wire. The shaded part of each figure represents 

 the hydrogen in the molecule, and the unshaded portion the group 



FIG. 97. The passage of an Electric Current in a Simple Cell. 



of elements, SO 4 , which complete the constitution of the sulphuric 

 acid molecule. The third row shows the result of the series of 

 decompositions which take place when the plates are joined by a 

 wire. At the zinc plate A zinc sulphate is shown formed, and at 

 the copper plate B free hydrogen is represented. 1 



1 The student must, however, in view of more recent work on the subject, avoid 

 picturing the process as taking place with the regularity indicated in the abovo 

 explanation and in Fig. 97. It is now believed, that most probably, the solution 

 of acid contains not only B^SC^ molecules, but also uncombined hydrogen atoms 

 (or as we shall learn to call them ions) and uncombined 804 groups. These ions 

 are thought to be electrically charged, those of hydrogen positively and the 04 

 groups negatively. Consequently, whenever these separate ions combine, the result- 

 ing HoSO4 molecule is electrically neutral. Each molecule of HoSC>4 is being con- 

 stantly decomposed into its ions, and these separated ions are constantly recorn- 

 bining in other cases to form new H 2 SO 4 . The net result of these decompositions 

 and recompositions is that the ratio of decomposed to undecomposed molecules 

 remains constant. The result of the union of the zinc with the negatively electri- 

 fied SO.i ions is to leave it negatively electrified. The preponderance of positively 

 electrified hydrogen ions in the liquid causes it to be positively electrified. Or, 

 using the word " potential," which the student has learnt, the zinc is left at a 

 lower potential than the liquid. But the zinc is in metallic connection with the 

 copper, which must also be at a lower potential than the liquid. The positively 

 electrified hydrogen ions hence travel towards the copper plate, where after giving 

 iip their positive charges they escape as gaseous hydrogen. By this means the 

 current is maintained. 



We should, perhaps, be more accurate if we regarded the negative electrification 

 of the zinc as due, not to the union with it of the negative tiO ions, but to the 

 departure from it, into the solution, of positive zinc ions. 



