MR. DAN1ELL ON VOLTAIC COMBINATIONS. 
113 
The next variation of the standard elements of the battery was to place conducting 
plates in connexion with one another on each side of the generating plates. This was 
easily effected by making both the plates, ij, Im, (fig. 3.) of platinum communicate 
with one mercury cup (p) ; while the amalgamated zinc plate placed between them 
communicated with the other cup (o). This arrangement was perfectly analogous in 
principle to the double or Wollaston plates of the common battery. Upon combining 
the cells thus arranged in single series, nearly as much gas was collected from each 
plate, on the opposite sides of the generating plates, as had been collected in an equal 
time from the single plates ; the double surface of the platinum having enabled the 
single zinc plate to decompose very nearly a double portion of water. 
During this experiment, and its frequent repetitions, I remarked that the bubbles 
of hydrogen arose not only from the surfaces of the platinum which were immediately 
opposed to the zinc, but from the opposite surfaces likewise ; and when a little sul- 
phate of copper was added to the liquid in the cells, both faces of the plates became 
coated with the reduced metal. 
Almost the only serious difficulty with regard to your chemical theory of voltaic 
action which ever occurred to me, has arisen from the well-known case of voltaic pro- 
tection, in which a small piece of zinc or iron has been found to defend from corrosion 
a surface of copper so many hundred times its own superficial dimensions. I was 
unable for a long time to understand how the hydrogen, which could only be the 
equivalent of the oxygen taken up by the former metals, became spread over so dis- 
proportionate a surface ; and this effect seemed to me more intelligible upon the 
hypothesis of some condition of the copper, analogous to those produced by electrical 
or magnetic induction, than upon any known chemical principles. The following 
means of determining the question by experiment now occurred to me. 
I took a silver plate, fifteen inches square, and, placing it in a shallow trough, 
covered it with the dilute sulphuric acid, to which a portion of sulphate of copper 
had been added. I then supported an amalgamated zinc wire of about one eighth of 
an inch in diameter, so as to allow one of its ends just to rest upon the centre of the 
plate. The instant the two metals came in contact, a circular spot of metallic copper 
was thrown, down upon the silver, and rapidly spread itself in such a way that in a 
few hours it formed a well-defined circle of six inches in diameter. Particles of cop- 
per could be traced even beyond this ; but none appeared to have reached the edges 
of the plate. 
I varied this experiment by soldering to the centre of a silver plate, 8 inches by 
5-J- inches, a small piece of amalgamated zinc ^-ths of an inch long by -\-ths wide, and 
placed it perpendicularly in a jar, and covered it with the solution of copper in acidu- 
lated water. The copper immediately began to precipitate itself upon the silver, in 
the form of an oval surrounding the zinc, and gradually extended itself equally on all 
sides. There was no extension of the copper on the upper side, to indicate that the 
hydrogen, the reducing agent, had been carried upwards by its levity ; but it spread 
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