706 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



When the current is established, the sulphate of copper is decom- 

 posed and a deposit of the metal is formed on the surface of the 

 mould. But in proportion as this deposit is formed, the bath is 

 impoverished by the same amount, becoming more and more acid, and 

 the deposited metal loses its plastic properties and its coherence, 

 unless the solution is maintained in its normal state of saturation 

 by crystals of sulphate of copper placed in a bag within the bath. 



What is called in electro-plating a compound apparatus only differs 

 from the simple apparatus in having the battery separate from the 

 bath; to prevent the impoverishment of the bath a sheet of copper 

 is suspended in it in communication with the positive pole of the 

 battery, while the mould is metallically united to the negative pole. 



FIG. 455. Compound apparatus for ele.ctro-plating. 



This sheet constantly gives up to the solution the quantity of copper 

 that has been deposited, so that the concentration of the bath remains 

 constant. Jacobi, to whom this latter arrangement is due, has called 

 the sheet of copper in the compound apparatus, the Soluble Electrode. 



We may now enter into certain details concerning the different 

 industrial and artistic applications of electro-typing. 



The processes just -described are applicable in that form to 

 the reproduction of medals, seals, and other objects of small 

 dimensions engraved on one side only. They are used for the repro- 

 duction of wood, steel, and copper engravings, which would rapidly 

 wear out and be spoiled, if submitted to direct working off, but 

 which electro-typing enables us to preserve indefinitely. 



A wood engraving gives at a maximum ten thousand copies. But 



