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 
eovers 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 th 
Jayers applied by the electro-chemical process. 
The electro-chemical appearances are formed 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 ta 
the production of the phenomenon of thin plates that the electro- 
negative elements should be precipitated on the metal with a certain 
ea 
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substances to enter into combination with them. This idea, which accords wit 
the spirit of other theories, being admitted, we see at once how these layer 
reserve 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 t 
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 that 
the attempt to evade it by the adoption of the term subomide, which is quite a: 
vague and undefined as the principle of oxidation, for which it was espa as 
substitute ? 
