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 mentioned. 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 beautiful 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-negative, 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 rapidity, 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 phænomenon 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 with 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, perhaps, 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?
