150 ELECTROLYSIS. 



and silver. The argentic solution is poured as long as the clots 

 freely disappear, and the solution is ready for use when the 

 precipitate cannot be dissolved any more. With this solution 

 a silver deposition may be obtained from a simple whitening up 

 to a very solid dead polish coating. 



As the bath grows weaker in silver, some fresh argentic 

 nitrate is added, and when the bisulphite has become too weak 

 to dissolve the argentic nitrate it is sufficient to pour some into 

 the bath in order to give the solution its original properties. 



2. GOLD-PLATING. 



COMPOSITION OF BATHS FOE GOLD-PLATING. Mr. Watt 

 indicates five formulae for gold baths used in the United States ; 

 they do not differ much from each other, and we will describe 

 two of them. 



1st Solution. Dissolve in a Florentine vessel 2 '33 grammes 

 of fine gold in aqua regia (2 parts of hydrochloric acid and 

 1 part of nitric acid). Pour the solution into a porcelain cup 

 and evaporate the acid ; there remains a reddish mass of auric 

 chloride. Dissolve this chloride, cold, in 30 grammes of dis- 

 tilled water. Add a concentrated solution of potassic cyanide, 

 stirring with a glass rod, until the gold is precipitated. Decant, 

 lixiviate the precipitate, and add some potassic cyanide for re- 

 dissolving the precipitate. Evaporate to dryness in a sand 

 bath the solution of auric cyanide, dissolve again the residuum 

 in cold water, and filter. For use, add some boiling distilled 

 water so as to make about 1 * 1 litre, and a little cyanide if the 

 bath acts too slowly ; but too much cyanide must not be used, as 

 it would attack the anode and give the deposit a bad coloration. 



2nd Solution. Dissolve as before 2 '33 grammes of fine 

 gold and evaporate. Eedissolve in 30 grammes of distilled 

 water and precipitate the gold with ammonia, avoiding excess. 

 Decant and lixiviate the precipitate. Dissolve the precipitate 

 with potassic cyanide, evaporate to dryness, and dissolve again, 

 when cold, in distilled water. Filter, and add some distilled 

 water to make about 1*1 litre. Add a small quantity of 

 cyanide to the solution. 



