310 Mr. E. Solly, Jun., on the Precipitation of 
perfect contact with the surface on which it is thrown down; and 
thus we here have the first element of the process of copying, 
or obtaining a cast by precipitation, but extremely crude and 
imperfect. The film of copper so thrown down would be so 
thin and fragile that it would be impossible to remove it from 
the surface of the tin; or if it should acquire any thickness it 
would of course do so at the expense of a considerable quan- 
tity of tin, because for every portion of copper precipitated 
a corresponding quantity of tin must have been dissolved ; the 
smoothness of the surface would become destroyed ; and as 
this corrosion does not act equally all over the surface, if any 
design had been traced upon it, it would be much weakened, 
and in some parts wholly obliterated. 
Both these difficulties are easily overcome in consequence 
of the facilities which electricity gives us, of, as it were, making 
chemical action portable ; of generating it in one place, convey- 
ing it along metallic conductors, and making use of its power 
in another. Hence we are enabled to employ at pleasure one 
of the most powerful known deoxidizing agents, hydrogen. 
By means of electricity, we are able to give to any piece 
of metal, the power of evolving hydrogen, under the most 
favourable circumstances, from its surface, whilst immersed 
in a cupreous solution ; and to continue that action, until the 
coat of deposited copper, precipitated by the action of the 
nascent hydrogen, has acquired any degree of thickness. 
In order to effect this, a piece of some highly oxidizable 
metal, such as zinc, is connected by a wire soldered to its one 
end with the plate of lead, tin, or other metal, on the surface 
of which is engraved the design proposed to be copied. A 
vessel of any shape and material is divided into two portions 
by a partition or diaphragm of membrane, unglazed earthen- 
ware, or any other porous substance; the one division being 
filled with a strong solution of sulphate of copper, and the 
other with dilute sulphuric acid. The zinc is then placed in 
the dilute acid,and the mould or form to be copied, in the 
solution of copper. The zinc continues to dissolve in the 
acid, and generate the power, which being conveyed through 
the solutions to the surface of the mould, there causes the 
precipitation of copper, whilst the wire joining the zinc with 
the metallic mould, forms the connexion necessary to com- 
plete the galvanic circle. 
The apparatus for this purpose is exceedingly simple, 
cheap, and easy of management ; and when once arranged, 
and set in action, requires no further attendance until the ope- 
ration is complete, when the mould is to be removed and 
the copy separated from it. 
