712 THE APPLICATIONS OF PHYSICAL FORCES. [BOOKV. 



a thin layer of oxide which must be got rid of by pickling, an 

 operation which is performed by dipping the object in a basin of 

 acid ; lastly, if the object to be gilded or silvered is of iron, steel, zinc, 

 or aluminium, it must be previously covered by electro-plating with a 

 thin coat of copper, without which the gold or silver deposited on 

 its surface will not adhere. 



And now as to the preparation of the bath. For gilding, a solution 

 of cyanide of gold in an excess of cyanide of potassium is used, and 

 for silvering the composition is similar namely, a solution of cyanide 

 of silver in an excess of cyanide of potassium. It is advisable to 

 keep the temperature of the bath, during the operation, above the 

 ordinary generally at 70 C., since when formed in the cold the 

 colour of the deposit is not so good. The positive pole is formed by 



FIG. 459. Compound apparatus for electro-silvering. 



a plate of gold or silver by which the current enters the solution, 

 and which acts as a soluble anode. The object to be silvered or 

 gilded forms the negative pole. When the electrolytic action commences 

 the cyanide of gold is decomposed, the gold is set free at the negative 

 pole, where it spreads by degrees all over the surface of the object ; 

 but the cyanogen, passing to the positive pole, combines there with 

 the gold, and as much cyanide of gold is formed again as the current 

 decomposes. The strength of the solution is unchanged, which is an 

 essential condition of the operation. The phenenoma are exactly 

 similar in the silver bath. 



Figs. 459 and 460 show how the apparatus for gold and silver 

 plating are arranged. The objects are suspended in it by copper rods 



