Current, and on the Intensities of Voltaic Arrangements. 109 



changed simultaneously. They always began to evolve oxygen 

 at about the same time; and when one of them ceased to 

 evolve oxygen and began to be oxidized and dissolved, the 

 same thing happened to the other at the same instant. 



When both of the iron electrodes were evolving oxygen, it 

 was only necessary to lift one of them up a little, so as to ex- 

 pose a small portion of its surface to the air, and then to plunge 

 it into the acid again, in order to make both irons instantly 

 assume the opposite state. The same effect was also produced 

 by touching the immersed portion of one of the electrodes 

 with a piece of iron or zinc. 



Now, as far as regards one electrolytic cell, the above phe- 

 nomena can be explained, I think, without much difficulty. 

 Adopting the theory of Professor Daniell, which, agreeably 

 to the theory of salts which has been promulgated by Davy 

 and Graham, supposes the positive metal to unite directly 

 with oxysulphion (S0 4 ), we can readily perceive that oxygen 

 must inevitably rise from the iron, whenever the oxysulphion 

 cannot be produced as quickly as is demanded by the inten- 

 sity of the battery. On the other hand, there will not, I think, 

 be much difficulty in admitting that the evolution of oxygen 

 may, by producing currents in the liquid, &c, have the effect 

 of restoring to the iron its original aptitude for dissolution ; 

 then, if the smallest portion of iron assume that state, it is evi- 

 dent that that portion will be positive with regard to the rest 

 of the iron evolving oxygen ; a current therefore will be esta- 

 blished through the acid from the former to the latter, and 

 the hydrogen thereby liberated immediately uniting with the 

 nascent oxygen of the passive portion of the iron, the whole 

 surface of iron will suddenly become clean and again combine 

 with oxysulphion. According to this view the advance of the 

 needle from its smallest to its greatest deflection ought to be 

 very sudden. This accords with my experience. 



The simultaneous change of the state of two iron electrodes 

 in separate cells between which the current of the battery is 

 divided, may perhaps be explained by supposing that, when 

 one of the iron electrodes enters into the active state, the 

 sudden increase of the intensity of the current through its cell 

 diverts the current from the other cell to such an extent as to 

 allow its iron electrode also to assume the active state. 



In general a current of a certain degree of intensity is re- 

 quisite in order to produce the intermittent effects. If it be 

 too low, the iron will continue to be dissolved ; if too high, the 

 iron will, after the first few moments of action, commence, and 

 then continue to evolve oxygen. A great deal seems also to 

 depend upon the quality of the iron employed. With some 



