ON THE SCIENCE OF ELECTRO-PLATING. 
43 
about two years before bright silver, and occurred whenever a 
large number of phosphorized wax moulds were put into a solution 
of sulphate of copper to receive an electro-deposit of copper. 
Other substances possess the quality of imparting brightness to 
deposited silver, but none in so satisfactory or eminent a degree 
as bisulphide of carbon : among these may be mentioned bicar- 
bonates of the alkalies, and many organic compounds, and it is 
probable that the brightness depends upon a gaseous body being 
dissolved in the plating liquid. 
No important improvement in the electro-deposition of silver 
has since been made ; and the process at present in use may 
be briefly described as follows : — A certain quantity of pure or 
virgin silver in a granulated state is taken, allowing about one 
ounce for each gallon of plating- solution required (the actual 
proportions, however, in use by manufacturers vary from a 
quarter of an ounce to two or three ounces of silver per gallon), 
a warm mixture of four parts of pure and strong nitric acid 
and one part of water, contained in a capacious vessel of glass 
or stoneware, is placed in a warm situation, where the air-fumes 
may readily escape without injuring persons or furniture ; and 
small quantities of the silver are added, from time to time, as 
fast as it dissolves, care being taken not to add it in too large 
quantities at a time — otherwise waste will ensue — until nearly 
all the silver is dissolved. It is advisable to employ a deficiency 
of the acid mixture in the first place, and to add more of it 
towards the end of the process, taking care not to add more 
than will dissolve all the silver, but rather to leave a little silver 
undissolved, even with the liquid quite hot ; by this means, the 
presence of much free acid is avoided, and an after-loss of 
cyanide of potassium and escape of poisonous fumes of prussic 
acid prevented. Each ounce of silver requires nearly one and 
three-quarter ounces of strong nitric acid to dissolve it. 
The solution of nitrate of silver obtained is now considerably 
diluted with distilled water in a capacious vessel of stoneware, 
and there is added to it, in portions at a time, with stirring, a 
solution of cyanide of potassium of moderate strength as long 
as a white precipitate or cloud, which is cyanide of silver, is 
produced; this precipitate is allowed to subside between each 
addition, and it is veiy particular that as the precipitate pro- 
duced becomes less copious the cyanide of potassium solution 
should be added more sparingly and at longer intervals of time, 
and that on no account should that liquid be added after it 
fails to produce a precipitate. This point requires some care and 
experience, but may be known by the cyanide of potassium 
solution producing a transparent but slightly brown appearance 
where it passes into the white and cloudy liquid : this trans- 
parency is caused by its dissolving the suspended fine particles 
