68 



Dr. E. Frankland. 



[Mar. 1, 



I have observed that during the so-called " formation " of a storage 

 cell, a very large amount of sulphuric acid disappears from the liquid 

 contents of the cell : indeed sometimes the whole of it is withdrawn. 

 The acid so removed must be employed in the formation of insoluble 

 lead sulphate upon the plates which, in fact, soon become coated with 

 a white deposit of the salt, formed equally upon both positive and 

 negative surfaces. This visible deposit is, however, very superficial, 

 and does not account for more than a very small fraction of the acid 

 which actually disappears from solution. The great bulk of the lead 

 sulphate cannot be discovered by the eye, owing to its admixture with 

 chocolate-coloured lead peroxide. 



Unless the coated plates have been previously immersed for several 

 days in dilute sulphuric acid, this disappearance of acid during their 

 "formation" continues for ten or twelve days. At length, however, 

 as the charging goes on, the strength of the acid ceases to diminish 

 and soon afterwards begins to augment. The increase continues until 

 the maximum charge has been reached and abundance of oxygen and 

 hydrogen gases begin to be discharged from the plates ; that is to 

 say, until the current is occupied exclusively, or nearly so, in the 

 electrolysis of hexabasic sulphuric acid expressed by Burgoin in the 

 following equation : — 



Eliminated on Eliminated on 



+ plate. — plate. 



S0 6 H 6 = S0 3 + 30 + 3H 2 . 



Sulphuric acid. Sulphuric anhydride. 



Of course the sulphuric anhydride immediately combines with water 

 and regenerates hexabasic sulphuric acid : — 



S0 3 + 30Ho = S0 6 H 6 . 



On discharging the cell, the specific gravity of the acid continually 

 decreases until the discharge is finished, when it is found to have sunk 

 to about the same point from which it began to increase during the 

 charging. Hence it is evident that, during the discharge, the lead 

 sulphate, which was continuously decomposed in charging, was con- 

 tinuously reformed in discharging. 



The chief if not the only chemical changes occurring during the 

 charging of a storage battery, therefore, appear to be the follow- 

 ing : — 



1st. The electrolysis of hexabasic sulphuric acid according to the 

 equation already given. 



■2nd. The reconversion of sulphuric anhydride into sulphuric acid. 

 3rd. The chemical action on the coating of the + plate. 



SO^Pb + + 30H 2 

 Lead sulphate. 



= Pb0 2 + S0 6 H 6 . 



Lead peroxide. Hexabasic sulpliuric acid. 



