124 ELECTROLYSIS. 



to understand that the layer of liquid which surrounds the 

 object grows weaker in metal and must frequently be replaced 

 by a newly saturated layer. In a large factory the best 

 arrangement consists in giving a continuous motion to the 

 liquid by means of an automatic apparatus. 



It sometimes happens that a piece to be nickel-plated is of 

 exceptionally large dimensions compared to the sizes of the 

 pieces generally plated ; if the anodes are smaller than the said 

 piece, and if no larger ones can be procured, or if the piece is of 

 a very irregular profile, a sufficiently considerable distance (at 

 least 30 centimetres) between it and the anodes should be main- 

 tained, and the operation should be very carefully watched. 



If, for example, notwithstanding these precautions, the coat- 

 ing does not take place at the bottom of a cavity, a plate of 

 nickel, or even of carbon, in communication with the negative 

 pole should be introduced in the interior of that cavity. 



When a large number of pieces are being plated at the 

 same time, care must be taken that these pieces do not cover 

 each other in the bath, and also that they are equally dis- 

 tributed between the anodes. 



It is necessary, owing to the low conductivity of nickel solu- 

 tions compared to that of gold and silver baths, to place the 

 anodes on each side of the piece, otherwise the deposition will 

 only be partial. For the same reason the nickel does not easily 

 travel round angles ; it is deposited freely enough on flat sur- 

 faces or on projecting parts, but with much more difficulty on 

 concavities, internal cracks, and inside angles. This resistance 

 to the deposition is rendered more manifest in the nickeling of 

 cast-iron pieces, in which there are a number of holes resulting 

 from the cores used for moulding purposes. The only means 

 of giving a good appearance to those pieces is to first cover 

 them with a metallic coating in a bath having a better con- 

 ductivity than nickel solutions : a coating of copper for example. 



Although it is often advantageous to have a thick coating 

 of nickel, some good operators cannot obtain it without altering 

 the deposit which cracks and peels off in large scales when 

 it reaches a certain thickness. This phenomenon takes place 

 when the pieces are receiving the last touch before being 



