PROFESSOR DANIELL ON THE ELECTROLYSIS OF SECONDARY COMPOUNDS. 211 
combine, and a portion of the copper remains in the metallic state, and a portion of 
gaseous oxygen escapes. The precipitation of blue hydrated oxide doubtless arose 
from the mixing of a small portion of the two solutions. 
Experiment 19. — The experiment was repeated with a smaller electrolytic force, 
six of the cells of the small constant battery being substituted for the twenty cells of 
the large. The action was of course much less energetic, and the membrane was 
found thickly coated with black oxide of copper ; amongst which, however, spangles 
of metallic copper were plainly visible. In this case there was time for the local 
affinity to act, and the combination of the copper and oxygen was nearly complete. 
Independently of the important conclusion which these experiments confirm, 
namely, that in the electrolysis of a saline electrolyte, the metal travels as a metal 
towards the platinode, while all the other constituents of the salt pass, as a com- 
pound anion, to the zincode, they present the secondary action of the local affinity in 
striking contrast with the primary action of the current affinity in the voltaic battery: 
all the decompositions and recompositions of the latter, however varied and multi- 
plied, are limited and equalized throughout the circuit, however long, by the strictest 
laws ; while the combinations of the latter vary with every variation of time, quan- 
tity, and energy. 
The experiment with sulphate of copper was repeated and varied in several ways ; 
as by placing the solutions in a cell separated by a perpendicular, instead of a hori- 
zontal diaphragm, &c., the particulars of which I need not dwell upon ; the results 
being always conformable to those which I have just detailed. 
I next submitted other saline solutions to the same electrolytic process, the second 
electrolyte being in every case solution of potassa. 
Experiment 20. — From nitrate of silver, metallic silver was deposited in abundance 
upon the membrane, mixed with oxide of silver ; gas was also visibly disengaged from 
the diaphragm. The whole of the oxygen was not evolved at the zincode, a portion 
appearing to combine, by secondary action, with the oxide of silver in the nitrate, and 
forming a peroxide. It was precipitated as a brown powder, which dissolved in nitric 
acid with evolution of oxygen. 
The experiment was varied by placing the solution of nitrate of silver in a small 
porous earthenware cell, which, upon examination afterwards, was found studded with 
minute filaments of silver, and coloured by the oxide ; but this apparatus was not 
found to be so well adapted to the purpose as the membrane, which, from its thinness, 
exhibited the results of the necessarily superficial action to perfection. 
Experiment 21 . — Nitrate of lead afforded similar results. Metallic lead was de- 
posited upon the bladder with oxide of lead, and, as with the silver, the oxygen was 
absorbed at the zincode with the precipitation of peroxide of lead. 
Experiment 22. — Protosulphate of iron threw down a copious black precipitate 
upon the diaphragm, which, when examined with a lens, exhibited bright metallic 
points ; but oxidation took place so rapidly, after the membrane was withdrawn from 
the cell, that the metallic lustre quickly disappeared. 
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