CHAP, ix.] ELECTRO-PLATING. 715 



of precious metal to be deposited on the submerged objects, equi- 

 librium is broken, and the beam dips on the right ; a metal rod with 

 which it is provided dips into a cup filled with mercury connected 

 with the negative pole of the battery, and then the circuit is closed 

 and the operation commences. 



The operation continues without supervision so long as the deposit 

 does not exceed the determined weight, but as soon as this limit is 

 passed, equilibrium is re-established, contact ceases, and the current 

 is interrupted. 



We need not enter into the details of the purely technical operations 

 which follow the deposit of the layer of gold or silver on the objects, 

 after they have been taken from the bath. We will only mention 

 that the dull colour of this layer is made brilliant by scratch-brushing 

 and burnishing ; that is to say, by rubbing the parts which ought to 

 be polished, by a rapidly rotating brass-fibred brush, and then 

 hand-rubbing by the workman by means of hard particles of stone 

 or steel mounted on rubbers. 



Silver is made to shine directly by placing in the bath, during 

 the operation, a very small quantity of sulphide of silver. This 

 process was invented by M. Plante. 



The electro-chemical method of silvering and gilding is now applied 

 on a very large scale all over the world. By its means has been 

 introduced into houses of very modest pretensions, the luxuries of the 

 well-to-do. 



The following extract from the Grandes Usines by M. Turgan 

 will show the importance of this industry in France alone : " A 

 few figures taken at random, will give an idea of the importance 

 acquired by electro-metallurgy in the house of Messrs. Christofle 

 since the expiration of Elkington's patents. In 1865 they silvered 

 5,600,000 objects, which has withdrawn from circulation 33,600 

 kilogrammes of silver, worth 6,700,000 frames ; an equal quantity 

 of objects executed in solid silver would have withdrawn from 

 circulation a million kilogrammes of silver; that is to say, more 

 than 200 millions of francs ; 33,600 kilogrammes of silver of 

 the thickness adopted in plating, that is three grammes on 

 each centimetre square, would cover an area of 112,000 square 

 tnetres." 



Gold and silver plating are now applied in a variety of circum- 



