130 ELECTROLYSIS. 



The acids must be contained in a stoneware vessel which is 

 placed under a flue provided with a good draught ; the water 

 and the potassic nitrate are added gradually. The pieces to be 

 scoured are suspended from a copper wire and first dipped into 

 boiling water, then quickly in the acids during half a minute. 

 They are taken out from time to time, the operation being con- 

 ducted with great caution, until the whole of the nickel coating 

 has disappeared. The pieces thus taken out of the scouring 

 bath must, each time before they are put back in it, be dipped 

 in cold water ; a large tub of water should be placed near the 

 operator. 



The pieces must then be thoroughly rinsed in hot water, 

 dried and polished with great care, and put back in the nickel 

 bath. With copper or brass pieces, if the scouring has been 

 properly carried out, the subjacent metal should show only 

 feeble traces of having been attacked by the acid solution. It 

 is with a view of preventing this that sulphuric acid is intro- 

 duced in the solution, and that a proportion of nitric acid only 

 strictly sufficient for acting on the nickel is used. 



The nickel can also be taken off the objects by means of 

 the electric current by employing these objects as anodes. In 

 this case a special bath should be used, for the solution might 

 get spoiled by the attack of the subjacent metal, and would not 

 be afterwards suitable for a good nickel-plating. 



DURATION OF BATHS. A nickel bath may be preserved 

 without alteration for several years. The only precaution to be 

 taken, when the bath is not in activity, is to provide it with a 

 tight-fitting cover to prevent the access of dust. In summer 

 the loss of water by evaporation is made good by the addition 

 of a proportional quantity of distilled water at the time of 

 using the bath. 



If green crystals are deposited on the walls of the vat, it 

 is a proof that the solution is too concentrated, and requires 

 an addition of water. The dissolution of the crystals is facili- 

 tated by using a separate vessel containing boiling water and 

 afterwards pouring the whole into the bath. 



For getting back all the nickel from the old solutions, it is 

 sufficient to remove the nickel electrodes and replace them by 



