Points in Electrolysis. 5 



non-adjuvant. Some heat will be evolved, and the change 

 from oxide to sulphate will not appear as external electrical 

 work. On charging again there is only enough electromotive 

 force at that contact to cope with the attraction of zinc for 

 oxygen and not with the greater attraction for S0 4 . The 

 result is that the cell is not reversible. Any secondary or non- 

 adjuvant action in the cell thus means non-reversibility. We 

 thus have to deal not merely with Zn,S0 4 or ZnO,S0 3 aq, and 

 so on, but in a reversible cell, like a Daniell, we have to 

 consider the attraction of Zn for S0 4 ; the physical change of 

 Zn from a solid to a liquid state, which is generally included 

 in the " heat of combination/'' as given ; the solution of the 

 ZnS0 4 ; and perhaps such small matters as change of volume 

 of the cell. The copper salts must be dealt with similarly. 

 Even if the heat of solution were non-adjuvant the cell would 

 not be reversible. 



Since 1883 I believe I have been alone in holding that not 

 only is lead sulphate formed on both plates of a secondary 

 battery, as shown by Dr. Gladstone and Mr. Tribe in 1882, 

 but that its formation is the cause of the action of the cell, 

 and not a secondary reaction at all. That is to say, there is 

 no intermediate formation of PbO. To find out E e of a cell 

 we must open a sort of book. On one side put the heat of 

 formation of every compound formed on discharge, and on the 

 other of every compound broken up on discharge ; the balance, 

 including the physical changes, giving E c . For instance, in 

 a storage cell we credit the formation of PbS0 4 on the spongy 

 plate, including, of course, the change of the S0 4 into the solid 

 state, and on the peroxide plate we again credit PbS0 4 . Two 

 equivalents of H 2 are also credited. Two equivalents of 

 H 2 S0 4 -f-Aq are debited, also two equivalents of H 2 S0 4 , 

 and one equivalent of Pb0 2 , allowing for the 2 becoming 

 liquid. 



It has been repeatedly urged that as the formation of 

 PbS0 4 takes place on both plates it must cancel out. As in 

 the case of a Daniell cell we have to deal with the difference 

 between the heat of formation of two sulphates, it is assumed we 

 must also deal with differences in a secondary battery. A 

 little consideration will show, however, that the cases are 

 different. In a discharging Daniell sulphate of zinc is formed 

 and sulphate of copper is decomposed, so one goes on the 

 credit side, the other on the debit. In a secondary battery 

 sulphate is formed on both plates on discharge, and is there- 

 fore credited twice. 



The doctrine of the Conservation of Energy also teaches us 

 that electrolytic or nascent oxygen and hydrogen, which many 



