70 HTDROMETALLURGY. [ill. 



in Pjrometallurgy, as they constitute some of the necessary 

 steps for extracting metals from their ores and purifying 

 them. The present division embraces all other liquid metal- 

 lurgic processes, especially the wide-spreading branch of gal- 

 vanoplastics, together with etching metals and photography. 



When we say that carbon is the great reducing agent em- 

 ployed in metallurgy, we refer to its exclusive use from time 

 immemorial in furnaces, both as fuel and a reducing agent ; 

 but recent experiments have shown its reducing and decompos- 

 ing power even in solution. Refer, also, to the third division 

 of Chemics for its decomposing power. 



Deoxidation ly Carbon in the ivet way. — Schonbein has 

 ascertained that the persalts of iron and the salts of red oxide 

 of mercury may be reduced, by agitating their solutions with 

 finely powdered charcoal (as ignited lamp-black) to salts of 

 protoxide of iron, and of the black oxide of mercury xq- 

 spectively. 



On the reduction of salts of iron to the metallic state, see 

 Journ. Fr. Inst. (3) xix. 354, and Chem. Gaz. April, 1850. 



1. Galvanojylastics. — We have a few points to offer on the 

 general subject of galvanoplastics, by which metals are pre- 

 cipitated in the metallic state by a galvanic arrangement, on 

 surfaces previously rendered conductive. These processes, 

 chiefly confined to gold and silver, are fast replacing the more 

 ancient methods of plating, over which they possess great 

 advantages, economy of time and material, convenience, facility 

 for obtaining plating of any required thickness, &c. 



Cyanides. Solution of metals in cyanide of potassium. — 

 Eisner has described in the Journ. f. Pract. Chemie, vol. xxxvii. 

 1846, experiments on the solubility of various metals in 

 cyanide of potassium, the general results of which he thus 

 gives. He found that the metals employed might be divided 

 into two groups : those which do not dissolve, as "platinum, tin, 

 and mercury, and those which dissolve. The latter are again 

 divisible into two groups : those dissolving with the decomposi- 

 tion of water, as iron, copper, zinc, and nickel ; and those 

 unattended by the decomposition of water, as gold, silver, and 



