186 ELECTROLYSIS. 



The solution must contain about 65 grammes of metal for 

 10 litres of water. 



The current must not be too strong, otherwise the depo- 

 sition of the platinum would be effected in a state of black 

 powder. 



The platinum anode not being attacked by the cyanide, the 

 bath grows weaker, and its strength must be maintained by 

 adding some platinic chloride from time to time. 



DEPOSITION OF LEAD. 



FRENCH SOLUTION. Dissolve 10 grammes of litharge (prot- 

 oxide of lead) in 100 grammes of caustic potash for 2 litres of 

 distilled water. Use a lead anode, and add some litharge from 

 time to time. 



AMERICAN SOLUTION. Simply dissolve plumbic acetate or 

 nitrate in water. The lead is easily deposited when the solu- 

 tion is only slightly concentrated and the current weak. For 

 laboratory experiments an alkaline bath is used ; it is prepared 

 by precipitating the lead of plumbic acetate or nitrate by 

 potash, soda, or ammonia, and dissolving the precipitate in 

 potassic cyanide. 



CHROMOPLASTICS, OB COLOURED EINGS. Beautiful colora- 

 tions may be obtained with a battery or a Gramme machine, on 

 polished steel plates by means of preparations of lead salts. 

 M. Roseleur gives the following process : " The object to be 

 coloured, attached to the negative pole, is immersed in a cooled 

 bath of plumbite of soda ; a platinum anode is then gradually 

 dipped into the liquor without touching the object with it. 

 The latter is immediately observed to become coloured with a 

 variety of hues : yellow at first, and each of these colours grow 

 darker and completely change as the platinum anode is more or 

 less deeply immersed. These lighting effects, which are caused 

 by the greater or lesser thicknesses of the deposited plumbic 

 acid, can be indefinitely varied. 



Mr. Watt proceeds in the same manner, but uses plumbic 

 acetate instead of protoxide. 



If a piece of sheet copper cut in a star shape is attached 



