SILVER AND GOLD PLATING. 149 



is completely precipitated, and melting the precipitate after 

 drying it; it presents the serious inconvenience of emitting 

 extremely dangerous vapours of hydrocyanic acid. The second 

 is based on the evaporation of the solution to dryness, the fusion 

 until complete reduction of the metal, and the elimination by 

 lixiviation of the potassic cyanide. Mr. Sprague advocates a 

 third process, which appears to us to be superior to the two 

 just described ; it is as follows : 



" Place the solution in a large phial provided with a safety 

 funnel tube and with an escape tube connected by means of an 

 indiarubber tube to a large glass tube, the extremity of which 

 dips for about 15 millimetres in a solution of argentic nitrate 

 placed in another vessel. Add then gradually, and until a new 

 precipitate is formed, sulphuric acid through the safety funnel 

 tube, waiting for the effervescence to calm down, and shaking 

 the phial. Then by means of a sand bath heat the phial and 

 keep the solution boiling as long as a precipitate is formed in 

 the other vessel. This precipitate is pure argentic cyanide, 

 which if dissolved into potassic cyanide will re-constitute a new 

 solution. 



" The precipitate in the phial is also argentic cyanide, but 

 not pure, and can be reduced by zinc and hydrochloric acid. 

 The potassic cyanide which would have had to be replaced for 

 precipitating the silver is thus economised." 



SILVER-PLATING BY SIMPLE IMMERSION. There exist a 

 few solutions by means of which silver can be deposited with- 

 out the help of a galvanic current ; we will only describe one, 

 as this subject is outside the scope of our book, and will select 

 Koseleur's process as being the most efficient. 



The solution recommended by Eoseleur is composed of sodic 

 bisulphite, to which is added any silver salt, but preferably 

 argentic nitrate, until it begins to dissolve it with difficulty. 



It is prepared by filling a stoneware vessel with sodic 

 bisulphite three-fourths full, and pouring into it, stirring 

 it all the time with a glass rod, a moderately concentrated 

 solution of argentic nitrate in distilled water. The contact of 

 the two liquids gives rise to the formation of large white clots 

 of argentic sulphite, which, owing to the stirring action, the sodic 

 bisulphite destroys, transforming them into sulphite of sodium 



