SILVER AND GOLD PLATING. 145 



2. The silver nitrate is dissolved in a volume of distilled water 

 equal to ten to fifteen times its weight, and hydrocyanic acid 

 is poured in, which instantly produces an abundant white pre- 

 cipitate of argentic cyanide. It has been ascertained that the 

 quantity of hydrocyanic acid poured into the liquid is sufficient 

 when, on pouring a few more drops of it into the clear liquid 

 which is on the top of the argentic cyanide, the latter is not 

 further disturbed and no more precipitate is formed. 



The whole is then filtered through calico ; the argentic 

 cyanide remains on the filter, and the water, the nitric acid, and 

 the hydrocyanic acid which might be in excess, pass through it. 

 The precipitate remaining in the filter is then washed in two or 

 three waters. 



3. The argentic cyanide thus prepared is poured into the 

 vessel which is to contain the bath, and diluted in 10 litres of 

 water. Finally the potassic cyanide is added and is dissolved 

 at the same time that it dissolves the argentic cyanide. The 

 double cyanide of potassium and silver is thus obtained which, 

 as we have said, constitutes the best silver-plating solution. 



CONCENTRATION OF THE BATH. Concentrated solutions 

 give a more rapid deposit than weak solutions, but require more 

 care. The free cyanide must be nearly equal to half the weight 

 of the dissolved silver ; with a less quantity the bath is a bad 

 conductor ; with a larger quantity, the solution dissolves the 

 silver of the anode and even that already deposited on the 

 cathodes. Stanniferous alloys require much more free potassic 

 cyanide than copper, brass, or German silver. When the 

 anodes become coated with a greyish deposit, it is a sign that 

 the solution is too weak of cyanide. 



The solution being naturally more concentrated at the 

 bottom than at the surface of the bath, the objects to be plated 

 receive a greater thickness of deposit at the bottom than at 

 the top ; it is therefore necessary, in order to avoid this incon- 

 venience, to constantly stir the objects for the whole duration of 

 the operation. This motion is, in any well-organised factory, 

 mechanically obtained ; it is useful not only for silver-plating, 

 but also in every other galvanic operation. 



ANODES. The anodes must be of pure silver ; they must 



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