44 
POPULAR SCIENCE REVIEW. 
of cyanide of silver ; if by accident too much, cyanide of potas- 
sium has been added; a cautious addition of dilute nitrate of 
silver solution (for which purpose a little should be reserved) 
in a similar manner; will bring it back to the neutral or proper 
point : the whole is then well stirred and allowed to subside 
until the supernatant liquid is quite clear. Each ounce of 
silver dissolved requires nearly half an ounce of cyanide of 
potassium of ordinary quality to precipitate it. 
The supernatant liquid is then filtered through a calico bag, 
the sediment put into the bag, and the bag filled five or six 
times successively with spring water. A small quantity of 
hydrochloric acid is added to the filtered liquid to precipitate 
any dissolved silver (of which there is always more or less), in 
the form of chloride of silver : this is allowed to subside, the 
clear liquid is thrown away, and the sediment retained on 
account of its silver. 
Now, the wet contents of the filter are transferred to a capa- 
cious vessel, and to it is added, with constant stirring, a strong 
solution of cyanide of potassium until it is all dissolved, a 
memorandum of how much cyanide of potassium is used being 
made, because the amount varies greatly in dilferent cases and 
is dependent upon the quality of that substance. If the cyanide 
of potassium is of ordinary quality, each ounce of silver em- 
ployed will require about two or two and a half ounces of 
cyanide of potassium to re-dissolve it ; whatever the quantity 
required to re-dissolve the cyanide of silver may be, an equal 
additional amount should now be added to the mixture to con- 
stitute free cyanide, and sufficient water then added to dilute 
the solution to the proportion of one ounce of silver per gallon, 
or any other strength that may be desired : the solution now 
only requires to be filtered and it is ready for use.* 
In coating articles with silver by means of tlris liquid a 
voltaic battery is employed : the battery varies in its arrange- 
ment in different establishments and in different cases, but 
always contains dilute sulphuric acid and plates or bars of zinc. 
The battery most commonly used consists of a sheet of copper 
and a plate of amalgamated zinc immersed in a mixture of oil 
of vitriol and water contained in a large stoneware jar ; the 
zinc plate is connected by a copper wire with the articles to be 
coated, and the sheet of copper is connected by another copper 
wire with a sheet of pure silver, which is hung in the plating- 
solution near the articles. In this arrangement the electricity 
is generated by the action of the acid and water upon the 
surface of the zinc, and passes from the zinc through that 
* This process is illustrated in Plate V. fig. 2. See also explanation at 
the conclusion of article. 
