110 M. NOBILI ON COLOURS, AND ON A NEW CHROMATIC SCALE 



their origin now placed beyond the reach of doubt. It may be safely 

 laid down as a general proposition that the oxygen of the atmosphere 

 produces them, not, as is supposed, by oxidizing the surface of the 

 metal, but by becoming fixed in the form of a thin plate or film similar 

 to those of the electro-chemical appearances. 



Copper, tin, and bismuth are pure metals, and I know not any layer 

 by which they could be coloured, except that which has been just men- 

 tioned. Let a plate of copper be laid on a piece of red-hot iron : the 

 plate becomes gradually heated, and all at once exhibits the most beau- 

 tiful colours, but they disappear as suddenly. Before it becomes 

 coloured the plate has a metallic lustre ; it subsequently ceases to 

 shine, and becomes evidently oxidized. It is therefore at the moment 

 when the colours manifest themselves that the oxygen of the air 

 precipitates itself on the copper. In the next moment the chemical 

 combination is effected, which takes place whenever the action of the 

 heat is sufficiently prolonged. If the plate of copper be removed from 

 the red-hot iron as soon as the first indication of a change of colour is 

 perceived at any point, the process of coloration will then go on more 

 slowly, the copper will not be oxidized, and the oxygen, which would 

 produce this effect under a more prolonged action of the heat, now 

 covers the plate with a film, which adheres to it like a varnish, and by 

 its transparency produces the usual colours. 



The origin of the violet colour given to steel to prevent it from rusting 

 is the same. The layer however which produces this tint in the steel 

 does not perhaps consist solely of oxygen, as it does when the metals 

 are pure. Steel is a carburet of iron, and the oxygen of the air in 

 being precipitated on this compound, becoming combined with the 

 carbon in some manner or other might form the layer in question. At 

 all events the layer does not change its nature ; it is always electro- 

 hegative, and secures the metal from oxidation as effectually as the 

 layers applied by the electro-chemical process. 



The electro-chemical appearances are fonned with surprising ra- 

 pidity, and the colours developed on metals exposed to the action of 

 heat are produced with equal promptitude. It is therefore essential to 

 the production of the phsenomenon of thin plates that the electro- 

 negative elements should be precipitated on the metal with a certain 



substances to enter into combination with them. This idea, which accords witli 

 the spirit of other theories, being admitted, we see at once how these layers 

 preserve the transparency required to produce the coloured rings, and do not 

 attack the metal so long as they are kept at such a distance as to be unable to 

 combine with its particles. Berzelius was more sensible of the difficulty, per- 

 haps, than any one else : but would not an open avowal have been better than 

 the attempt to evade it by the adoption of the term suboxide, which is quite as 

 vague and undefined as the principle of oxidation, for which it was offered as a 

 substitute ? 



