534 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



copper) and an electrolyte (e.g. dilute sulphuric acid). The 

 pure anhydrous acids do not conduct and do not therefore 

 provide a proper electrolyte. Pure zinc, on the other hand, 

 fails to dissolve in dilute acids because, in the absence of 

 impurities, such as lead, arsenic and carbon, there is only 

 one electrode provided for the passage of the current. 



Armstrong (1885) regards chemical change as reversed 

 electrolysis. H. E. Armstrong, in 1885 (B.A. Report, Aber- 

 deen, 945-964), applied these ideas boldly to chemical 

 changes of every type. In electrolysis, an electrolyte is 

 decomposed by energy supplied from an external source : 

 in a battery, decomposition of the electrolyte is itself a source 

 of energy. The decomposition of the electrolyte taking 

 place in the single fluid cell results in the conversion of 



Zn + SO 4 |H 2 + Cu 

 into ZnSO 4 +H 2 + Cu. 



In the Daniell cell two electrolytes are decomposed in 

 series, the system 



Zn + S0 4 ]H 2 + S0 4 |Cu + Cu 

 changing to ZnSO 4 + H 2 SO 4 + Cu + Cu. 



In the Grove cell the change is from 



Zn + SO 4 |H 2 + ONO 2 H + Pt 

 to ZnSO 4 + H 2 O + NO 2 H + Pt; 



dilute sulphuric acid is electrolysed, but the concentrated 

 nitric acid in the cell is reduced, rather than electrolysed, 

 and may be regarded as constituting the second of the two 

 electrodes between which the electrolysis of the sulphuric 

 acid takes place. 



Armstrong, adopting a hint given by Faraday, suggested, 

 in 1885 that all chemical changes are electrolytic in char- 

 acter and take place under the same conditions that prevail 

 in a cell, namely, by the co-operation of three distinct substan- 

 ces, at least one of which must be an electrolyte. In the union 



