252 VOLTAIC ELECTRICITY. 



the current be reversed, silver will be deposited upon the copper 

 plate, which will thus be silver-plated. If the positive electrode be 

 a plate of gold and the bath a solution of some salt of gold (cyanide 

 of gold dissolved in a solution of cyanide of potassium), gold will 

 be deposited upon the copper of the negative electrode, which will 

 be thus electro-gilded. 



4OO. Electrotyping. Impressions are taken from 

 type or engravings in wax, or any other plastic material 

 that is impervious to water. A conducting surface is given 

 to such a mould by brushing finely-powdered graphite over 

 it, and it is then placed in a solution of sulphate of copper 

 facing a copper plate. The mould is then connected with 

 the -f plate of a Galvanic battery and the copper with 

 the plate ; when the circuit is closed, copper will be 

 deposited upon the mould. "When the copper film is thick 

 enough (say as thick as an ordinary visiting card) it is 

 removed from the mould, and strengthened by filling up 

 its back with melted type-metal. The copper film and 

 the type-metal are made to adhere by means of an amalgam 

 of equal parts of tin and lead. The copper-faced plate 

 thus produced is an exact reproduction of the type and 

 engravings from which the mould was made. 



e. It will be noticed that in all these cases the metal is 

 carried in the direction of the current and deposited upon the neg- 

 ative electrode. In electro-plating and gilding, the technicalities 

 of the art refer chiefly to the means of making the deposit firmly 

 adherent. In electrotyping, they refer chiefly to the preparation 

 of the mould or matrix. The countless applications of this process 

 of depositing a thin metallic coat on a body prepared for its recep- 

 tion, constitute the important art of electro-metallurgy. 



4O1. Electro -chemical Series. The facts just 

 considered suggest a division of substances into two classes, 

 electro-positive and electro-negative. The constituent 

 of an electrolyte that goes to the negative electrode 



