M. Nobili on Nezv Electro-Chemical Phenomena. 313 



a few seconds there are formed different concentric circles as 

 brilliantly coloured as the Newtonian rings. These rings rise 

 the one out of the other, propagating themselves like waves. 

 Their vivacity and distinctness depend in a great measure 

 upon the polish of the metallic disc, being weak and confused 

 on imperfectly polished surfaces. They resist the action of a 

 moderate heat, but they disappear entirely with nitric acid. 



From this and other circumstances there can be doubt that 

 these rings are thin plates deposited by the action of the elec- 

 tric current on the metallic surface. 



This phenomenon becomes more precise and more varied 

 when we multiply the points on the negative side, and ar- 

 range them in regular figures, as a triangle, square, &c. 

 There are formed on the disc as many systems of concentric 

 rings as there are points, but in place of intersecting each 

 other as they expand, like waves, they extend outwards when 

 they come in contact, so as to form only a single outline. This 

 appearance reminds us of the figures formed by sand upon vi- 

 brating plates, as described by Chladni, Paradisi, and Savart. 



A disc of Silver Positive presents also the coloured rings, 

 but with less distinctness. No remarkable effect is produced 

 by lead, tin, copper, bismuth, and antimony. 



Acetic Acid. — On Gold and Platina Positive there is on- 

 ly an uncertain colour, as with acetate of lead. 



Acetate of Copper. — >On Platina, Gold and Silver Po- 

 sitive nothing remarkable is observed, but it is otherwise when 

 they communicate with the negative pole. On Silver there 

 is often formed Jour concentric circles, which, when exposed 

 to the air, become a deep blue at the centre, then a yellowish- 

 red, then a less deep blue, and lastly, a yellowish-red shade, 

 forming a ring wider than the first. Nitric acid makes the 

 exterior circle disappear, but the three interior circles remain, 

 leaving the ordinary colour of copper in its two states of ox- 

 ide and metal. In the centre is the oxide, and then the pure 

 metal, surrounded with another circle of oxide. Platinum 

 and gold present analogous phenomena. 



Acetate of Potash, on Silver Positive, exhibits a dark 

 circle in the middle of other three which are four lines in 

 diameter, and surrounded with a very brilliant fillet of silver, 



