NICKEL-PLATING. 131 



carbon or platinum electrodes, taking care to add from time to 

 time some semi-liquid chalk and stirring frequently in order to 

 prevent the bath becoming too acid. If the nickel of the solu- 

 tion is not pure enough, the objects are replaced by carbon 

 plates, upon which the metal is very freely deposited ; the said 

 plates are afterwards dipped into nitric acid, which dissolves 

 the metal. This solution is then boiled in a porcelain cup 

 until complete dryness. 



Mr. Urquhart recommends the following process for the 

 extraction of nickel from old baths : " I avail myself," says he, 

 " of the curious property possessed by sulphate of ammonia of 

 precipitating the double sulphates of nickel and ammonium 

 from their solution. I therefore prepare a saturated solution 

 of sulphate of ammonium in hot water and add it to the old 

 solution, stirring it constantly. No result is at first to be ob- 

 served, but after the lapse of a few minutes, a deposit of double 

 sulphate begins to fall. The precipitated salt is of absolute 

 purity, and can be directly used for making a fresh solution. 

 This operation must be continued until the liquid has become 

 colourless." 



When the nickel solution has lost the emerald or greenish 

 blue colour which characterises it, and has, on the contrary, 

 become dirty, brownish or colourless, it will be well to cease 

 using it. 



If the pieces which are being plated are becoming yellow, 

 it is because the solution has become too alkaline, and con- 

 sequently requires to be neutralised by means of a weak acid, 

 as we have already indicated. 



M. Pfanhauser recommends, in order to avoid the coating 

 becoming yellow, the following process, which seems to us to 

 be good, and especially when a great tension is used : 



As soon as the objects have been taken out of the bath, they 

 must be immersed, without being rinsed, in a second bath 

 specially prepared for the purpose. This supplementary bath 

 must be acidulated by means of chemically pure hydrochloric 

 acid, or of nitric acid. The immersion must only last a few 

 seconds. The current, instead of being direct, will be reversed, 

 that is to say, the anodes will be connected with the negative 



K 2 



