COPPERING. 161 



but owing precisely to the said dissolution of zinc, the bath 

 after a certain time contains more zinc than copper, and must 

 be frequently renewed. The copper coating, always very thin, 

 often has a friable appearance, and becomes coloured with 

 iridated shades due to a slight oxidisation of the copper. 



M. Weil has perfected his process, and in a note recently 

 addressed to the Academy of Sciences he thus describes the 

 three methods which he generally uses : 



" The first method consists of dipping the pieces in the bath 

 in contact with the zinc wires. The coppering immediately 

 takes place, and afterwards secures the subjacent metal from 

 the attack of acids. It requires a time varying from a few 

 minutes to a few hours, according to the alkalinity of the bath 

 and the purposes for which the objects are intended. 



" The second method, which is used for coppering the lamp- 

 posts of large towns, consists in placing some porous cells in 

 the vat containing the copper alkaline-organic bath and the 

 objects to be coated with an average thickness. These porous 

 cells are filled with a lixivium of caustic soda, in which some 

 zinc plates connected by means of a large zinc wire to the 

 pieces to be coppered are immersed. The sodic lixivium can 

 be indefinitely used, for, as soon as it is nearly saturated with 

 oxide of zinc, it is treated by sodic sulphide, which regenerates 

 the caustic soda, at the same time precipitating some white 

 sulphide of zinc, which is sold at a good price. Coppering by 

 this method requires only a short time. 



"The third method applies to the coppering of various 

 objects of small, average, or very great thicknesses, by means 

 of the same baths and a dynamo-electric machine. 



" The baths only require from time to time the addition of 

 a given quantity of oxide of copper. When the copper is nearly 

 exhausted, it is titrated as follows : Put 10 cubic centimetres 

 of the bath solution in a white glass matrass, add 30 to 40 

 cubic centimetres of pure hydrochloric acid ; boil and pour the 

 greenish yellow solution on titrated stannic protochloride until 

 complete discoloration of the latter. The volume of stannic 

 chloride used for that purpose correctly indicates the quantity 

 of copper contained in the bath. There only remains to add 



M 



