of Silver and of Copper, 397 



inent which I have found to give the best results with silver 

 is extremely simple. Suppose a new sheet of silver is to be 

 used either for a cathode or anode — first make the corners and 

 the edges round and smooth, and then polish the surface 

 thoroughly with a soft clean pad, water, and fine silver sand, 

 so as to remove the skin which has been in contact w f ith the 

 rolls in the manufacture ; rinse the plate in clean water, or 

 by holding it in a rapid stream of clean water from a water- 

 tap, so as to remove the sand, and then wash it first with clean 

 soap and water and afterwards with clean water; next place 

 it for a few minutes in a boiling solution of cyanide of potas- 

 sium, and after that wash it thoroughly in a stream of clean 

 water, taking care not to touch the part of the surface which 

 is to receive the deposit with anything. If the surface of 

 the plate be touched with the fingers, even when the}' appear 

 to be clean, the markings of the skin will be reproduced by 

 the deposit leaving the parts of the plate bare which were in 

 contact with it. The plate may be dried either in a current 

 of dry air in front of a bright fire, or in any other convenient 

 manner which will insure that the surface of the plate will 

 remain clean, and then accurately weighed. One point about 

 weighing should be mentioned, because it is apt to be over- 

 looked, that is, that the plate must be allowed to assume the 

 temperature of the air before it is weighed, as even a very 

 slight difference of temperature between the plate and the air 

 inside the case of the balance is sufficient to produce quite a 

 sensible error in the weight. 



For the preparation of copper-plates a very good plan, 

 especially with large plates and powerful currents, where it 

 is necessary to arrange the resistance of the circuit carefully 

 when the electrolytic cell is included, is, after the edges and 

 corners of the plates have been well smoothed and rounded, 

 to proceed as follows : — Polish the plate thoroughly with 

 silver sand, wash the sand off by holding the plate in a rapid 

 stream of water and rubbing it with a brush or a piece of 

 clean cloth ; place the plate in the cell, as a trial plate, and 

 deposit a thin coating of copper over it, at the same time 

 adjusting the current to the proper strength ; then remove 

 the plate, wash it in clean water, and dry it, first in a clean 

 blotting-pad, and then before the fire, taking care not to heat 

 the plate sensibly. If the plate has not been properly cleaned, 

 the deposit of copper will show any defect; and if it be found 

 to be perfect, the plate may be weighed and the electrolysis 

 continued. For small currents it is more convenient to use 

 a battery of moderately high potential, say twenty or thirty 

 volts, and to keep a resistance in circuit with the cells; and in 



