CHAP. XL] ELECTRICITY AND MAGNETISM. 391 



base metals, such as pewter, they are usually first 

 copper-coated. The gilding of the insides of jugs and 

 cups is effected by filling the jug or cup with the gilding 

 solution, and suspending in it an anode of gold, the vessel 

 itself being connected to the pole of the battery. 





Fig. 157- 



Instead of a battery a thermo-electric generator (Art. 

 384), or a dynamo-electric generator (Art. 408), is now 

 frequently employed. 



422. Metallo-chromy. In 1826 Nobili discovered 

 that when a solution of lead is electrolysed a film of 

 peroxide of lead forms ttpon the anode. If this be a 

 sheet of metal, a plate of polished steel, for instance, 

 placed horizontally in the liquid beneath a platinum wire 

 as a kathode, the deposit takes place in symmetrical 

 rings of varying thickness, the thickest deposit being at 

 the centre. These rings, known as Nobili's rings, 

 exhibit all the tints of the rainbow, owing to interfer- 

 ence of the waves of light occurring in the film causing 

 rays of different wave-length and colour to be suppressed 

 at different distances from the centre. The colours 

 form, in fact, in reversed order, the " colours of thin 

 plates " of Newton's rings. According to Wagner this 



