106 M. NOBILI on COLOURS, AND ON A NEW CHROMATIC SCALE 



tions are far surpassed by those of the thin laminae. If we imagine one 

 of the colours of the lamince combined with another, we have the im- 

 pression of a new tint. The combinations that may be obtained in this 

 way are almost innumerable, and, it will be said, need well be so, in 

 order to match the variety which nature exhibits. Such is our opinion 

 too ; but we shall not attempt to conceal the difficulty presented by 

 the fact, that several of the natural colours, especially those of the me- 

 tallic substances, have but a very slight resemblance to the colours of 

 thin plates, among which it were vain to seek, for example, either the 

 yellow of gold or the red of copper. The colours of the plates which 

 approach them most nearly are found among the first seven or eight 

 tints of the scale. The gold might be placed among the blond colours, 

 and the copper among the tawny ; but the difference is still so striking 

 that it would be unwarrantable, before it is accounted for, to put entii-e 

 and implicit confidence in the principle of the laminae. 



This principle requires, as a primary condition, that the integrant 

 molecules of bodies should be transparent. It is true that almost all 

 bodies reduced to a certain degree of tenuity are permeable to light ; 

 but it is equally true that the existence of a single body perfectly opaque 

 and yet exhibiting a colour, would render it necessary to look for anr 

 other principle of coloration besides Newton's, which is applicable only 

 to diaphanous substances. 



In my Memoirs on the electro-chemical appearances, I have shown 

 that they are not exclusively produced by one of the poles of the pile. 

 The appearances which constitute the chromatic scale are due to the 

 electro-negative elements of the solution (oxygen and acid), which being 

 transferred by the current to the positive pole, are there spread out 

 into thin transparent films, from which all the colours of the scale arise. 

 The electro-positive elements (such as hydrogen and the metallic bases) 

 are, on the contrary, transferred to the negative pole, and there depo- 

 sited in layers which never produce the colours of thin plates. Here 

 it is impossible to mistake in any case, but more particularly in respect 

 to the solutions of certain salts with a base of gold or of copper, which 

 produce negative appearances invariably of the same colour as the me- 

 tallic base. It cannot be said in this case that the substance has not 

 been brought to the degree of tenuity necessary to render it transparent. 

 The electro-chemical layers commence with the first degree of attenu- 

 ation at the positive as well as the negative pole. If the layers of the 

 positive pole produce the ordinary colours of the plates, while the oppo- 

 site pole completely fails to present any other than that of the metallic 

 base, it necessarily follows, either that these bases are perfectly opaque, 

 or at least that their transparency is so imperfect as to render it im- 

 possible to apply the general laws to them, unless with very important 

 restrictions. Indeed we have here a decisive proof that the colours 



