146 ELECTROLYSIS. 



not be suspended from copper wires as those would become dis- 

 solved in the bath, but from iron wires or lead ribbons ; the 

 attachments are then joined to silvered copper conductors in 

 connection with the source of electricity. The anodes must be 

 entirely immersed in the bath, otherwise they would be eaten 

 at the surface, and the inferior extremity would drop to the 

 bottom. If the anodes are of common silver, they become 

 red owing to the formation of cupric cyanide, which alters the 

 purity of the deposit. The surface of the anodes must be 

 approximately equal to the total surface of the pieces to be 

 plated ; the distance between the anodes and the pieces to be 

 plated must be at least four inches. 



PKEPAKATION OF THE OBJECTS. The preparation of objects 

 for silver-plating is, generally speaking, simpler than for nickel- 

 plating, because the deposition of silver is effected as a rule on 

 objects which have previously been subjected to careful work- 

 manship, whereas the deposition of nickel is generally effected on 

 pieces which are rough from rolling or moulding. But the 

 operations are based on the same principle ; they comprise, for 

 copper, brass, and other similar alloys, the cleansing, pickling, 

 scouring, and amalgamation. 



1. Cleansing. The pieces are placed for a certain time in 

 a boiling solution of 10 litres of water and 1 kilogramme of 

 potash or caustic soda, then washed first in hot water and then 

 in pure tepid water. 



2. Pickling. The pickling is effected in a bath of 10 litres 

 of water acidulated with a tenth part of sulphuric acid. A 

 vigorous rinsing must follow. 



3. Scouring. The pieces are first passed through a pre- 

 liminary bath composed thus : 



Nitric acid at 36 degrees 10 kilogrammes. 



Sodic chloride 200 grammes. 



Lampblack 200 



then quickly rinsed in running water and dipped in the follow- 

 ing bath, prepared on the previous day and cooled : 



Nitric acid at 36 degrees 6 kilogrammes. 



Sulphuric acid at 66 degrees .. .. 200 grammes. 

 Sodic chloride 200 



