Current, and on the Intensities of Voltaic Arrangements. Ill 



have been able to prove, by the motions of the needle of the 

 galvanometer, that the disappearance of the film or white shade 

 is always accompanied by a sudden increase of the intensity 

 of the current. 



On dividing the battery current between two similar elec- 

 trolytic cells, I observed that the disappearance of the white 

 film occurred at the same moment on both of the positive 

 electrodes of amalgamated zinc. 



It is evident, therefore, that the phenomena obtained with 

 amalgamated zinc are, in a great measure, analogous to those 

 observed with iron ; but there is an important distinction be- 

 tween the two, inasmuch as no oxygen is evolved from amal- 

 gamated zinc when made positive in dilute sulphuric acid, 

 even when a very powerful battery is employed % So that we 

 see that amalgamated zinc continues to be dissolved even 

 when it has assumed a state analogous to that of passive iron 

 evolving oxygen. I think that this fact may be pretty well 

 explained by supposing that, in the active state, the zinc com- 

 bines immediately with oxysulphion, but that in consequence 

 of the too tardy arrival of that compound the zinc sometimes 

 combines with oxygen alone as a proper electrolytic action, 

 depending upon the secondary action of the sulphuric acid for 

 the removal of the film of oxide thus formed. It is easy to 

 see that in the latter case the intensity of the current will be 

 less than when the metal combines immediately with oxysul- 

 phion. 



P.S. — Nearly the whole of the above had been written 

 before I was aware that Schcenbein had already observed the 

 intermitting passivity of iron. As, however, my experiments 

 with the iron electrode are considerably different from those 

 of Schcenbein, I have not thought it right to suppress them. 

 The experiments of Schcenbein were made in the following 

 mannerf: — The conducting wires of a powerful voltaic pair 

 were connected with two mercury cups; a plate of platinum 

 immersed in dilute sulphuric acid was connected with the 

 negative cup; then a piece of iron wire, previously connected 

 at one of its extremities with the positive mercury cup, was 

 made to complete the circuit by immersing the other extre- 

 mity in the dilute acid. Under these circumstances he did 

 not observe any disengagement of hydrogen from the negative 

 platinum, in consequence of the passivity of the iron electrode. 

 He observes that the apparatus may be made to lose this state 



• Oxygen is evolved by zinc when the latter is made positive in a dilute 

 solution of potassa by three or four cells of Daniell in series. 

 f De la Hive's Archives de I' Electrwite, No. 5, p. 267- 



