134 ELECTROLYSIS. 



and allows the copper to more completely cover it, preserving 

 it from the spots which are difficult to avoid without taking 

 that precaution. 



It is also necessary to take special care of the casting of 

 pieces of zinc which are intended to be nickel-plated ; the grain 

 must be as pure and fine as possible, any abnormal cavity and 

 the vent-holes must be hermetically closed, the surface must 

 be perfectly homogeneous throughout, and all the pieces should 

 be carefully examined before being put into the bath. 



The recommendation never to suspend the pieces in the 

 bath without previously passing the current applies more par- 

 ticularly to zinc objects ; the best plan is to attach them to the 

 conductor and actually close the circuit by the immersion of 

 the object itself. At the beginning the current must be ener- 

 getic, then it is gradually weakened after the objects are first 

 whitened. The best arrangement would consist in having two 

 nickel baths; a setting one, traversed by a strong current, 

 with a large surface of anodes, and a finishing one with a low 

 current ; the two baths should of course be as neutral as pos- 

 sible to avoid the zinc being attacked. 



The couple formed by the nickel anode and the pieces of 

 zinc being pretty energetic, care should be taken to prevent 

 any reversal in the current by attentively watching the galva- 

 nometer. The use of the cut-out is almost indispensable in 

 such delicate operations. 



Laminated zinc can be nickel-plated with greater facility 

 than cast zinc, as we shall have occasion to explain in detail 

 when we describe the works of Messrs. Neumann, Schwartz, and 

 Weil; we shall only say now that the copper bath generally 

 used contains an excess of potassic cyanide (300 grammes 

 instead of 200). 



Meidingers Process. M. Meidinger condemns the cyanide 

 bath as being violently poisonous, and recommends the plating 

 of the zinc by means of amalgamation. 



As amalgamation renders zinc very brittle, it is necessary 

 to leave the zinc only for a short time in the mercurial solution. 

 If mercury is poured on a sheet of zinc previously scoured in 

 acid, it becomes so brittle that it cannot even be bent. 



