PLATING BY GOLD OR SILVER. 71 



cadmium. Solution is effected in both cases bj oxygen, but 

 in one it is evolved from water, in the other extracted from 

 the air. A part of the cyanide of potassium is oxidized to 

 potassa (hydrogen escaping when water is decomposed), and 

 the cyanogen, set free, unites with the metal ; the metallic 

 cyanide then forms a double salt with cyanide of potassium. 



Oxide of Crold. — Figuier (Journ. de Pharra. 1847), who 

 tested the several methods of preparing this oxide, now so 

 extensively used in electro-gilding, has determined the best 

 to be as follows. Dissolve 1 pt. gold in 4 pts. aqua regia, 

 evaporate to dryness, redissolve in water, add a little aqua 

 regia to take up the traces of metallic gold and of proto- 

 chloride remaining undissolved. Evaporate again, redissolve 

 in water, and mix with pure potassa perfectly free from 

 chloride, until it gives an alkaline reaction with turmeric paper. 

 Turbidity immediately ensues, when it is mixed with chloride 

 of barium ; — aurate of baryta precipitates as a yellow powder. 

 When the precipitate begins to assume a whitish appearance, 

 the addition of chloride of barium must be discontinued, as 

 all the gold oxide has gone down and the alkali commenced 

 to act upon the baryta of the chloride. The aurate of baryta 

 is then to be washed until the waste-waters cease to be pre- 

 cipitated by sulphuric acid. The aurate is then heated to 

 boiling, with dilute nitric acid, in order to eliminate the oxide 

 of gold. By washing until the water no longer reddens litmus 

 paper, the oxide becomes pure, and must be dried between the 

 folds of bibulous paper by exposure to air. 



Amalgamated Zinc. — Stoddard (Silliman's Journ. 1849) 

 has recommended the following method of amalgamating zinc 

 for cylinders for galvanic batteries. The zinc is heated to 

 450-500° and moistened with a solution of double chloride of 

 zinc and ammonium, and mercury immediately dropped over 

 the surface while still moist — the union is complete in a few 

 seconds. 



2. Plating hy Crold or Silver. — Both galvanic and other 

 methods are here included, as far as they have been recently 

 improved. 



