ON THE SCIENCE OE ELECTRO-PLATING. 
45 
liquid to tlie slieet of copper, then along the copper wire to the 
sheet of silver, then through the plating-liquid to the articles to 
he coated, and back to the zinc plate by the other copper 
wire. 
The electricity in passing from the surface of the sheet of 
silver into the plating-liquid causes the cyanide of potassium 
to act upon that metal and dissolve it, and at the same time 
the electricity passing into the surface of the articles decom- 
poses the solution in contact with them and causes it to yield 
up its silver to those surfaces; and these two simultaneous 
actions are perfectly equal in amount, i. c., for every ounce of 
silver dissolved on one side an ounce of silver is deposited on 
the other, and thus the amount of silver in solution remains 
unaltered for an indefinitely long period. The only alteration 
that takes place in the liquid is that it becomes unequal in compo- 
sition in different parts — that portion of it about the dissolving- 
plate becomes richer in silver and specifically heavier, and there- 
fore sinks to the bottom of the vat, whilst that about the articles 
becomes poorer in silver, specifically lighter, and rises to the 
surface; and this inequality renders it necessary to stir the 
liquid occasionally, otherwise the quality of the metal deposited 
upon the articles would be different at the upper ends to what 
it is at their lower ones. 
In most electro-plating establishments two or three such 
battery-cells as those described are used for depositing silver, 
and in the early period of plating a much larger number was 
used. When several cells are employed, the zinc plate of the 
first one is connected by a wire with the copper of the next, and 
so on throughout the series, leaving the extreme copper at one 
end and the extreme zinc of the other to be connected with the 
vat in the manner described : by this means there is a course 
opened throughout for the electricity to circulate, and each 
additional cell or pair of plates imparts an additional impulse 
to the electric current.* 
Several other kinds of voltaic batteries besides the one 
described are also extensively used in electro-deposition ; there 
is Smee’s battery, which contains a sheet of platinized silver 
instead of the sheet of copper; Grove’s, which consists of 
amalgamated zinc in dilute sulphuric acid and a sheet of 
platinum in strong nitric acid, the two liquids being kept 
from mixing freely, but allowed to touch each other by means 
of a separating diaphragm or cell of unglazed (i. e. porous) 
earthenware; and Bunsen’s battery, which is similar to Grove’s ; 
graphite (obtained from gas retorts) being, however, employed 
instead of the sheet of platinum. Dauiell’s battery, which con- 
* Plate Y. fig. 3. Zinc and silver are here used. 
