But this is not all: the superficial colours of which we speak are changeable, and belong evidently to the same class as those produced by thin plates. Now the pure metals, as we have already seen, are, from their opacity, incapable of this species of coloration. Can they acquire that capacity in their first degree of oxidation by becoming suddenly transparent in consequence of their union with a small quantity of oxygen? The hypothesis far exceeds the bounds of probability, and the phænomenon requires to be otherwise explained.
Let us return, for an instant, to the experiment of the coloured rings developed on a surface of platina by means of the electro-chemical apparatus described in the beginning of this Memoir. The platina surface belongs to the positive pole of the pile, and the electro-negative elements of the solution (which in the present case are the oxygen of water and the acid of acetate of lead) are deposited at this pole. I will not undertake to say by what species of affinity or force it is that these elements are attracted to each other and spread out into thin films on the platina. It is certain, however, that they attach themselves to the platina without oxidizing it in the slightest degree. We must not suppose that this happens because platina is a metal difficult to be oxidized. Iron and steel belong to the class of metals most easily oxidized, and yet it is well known that they will bear to be covered with electro-negative layers without becoming rusted. My electro-chemical experiments, multiplied and varied in a thousand ways, leave no room for reasonable doubt on this point: they show that oxygen and certain acids may adhere to the surfaces of metals without producing the slightest chemical change in them. This is a novel state for oxygen and the acids, and is distinguished from their ordinary combination by the three following peculiarities: 1st, The metal retains, beneath the deposited layer, its natural brilliancy; 2nd, this layer produces the phænomenon of the coloured rings in all its beauty; 3rd, instead of oxidizing the metal, these electro-negative elements contribute to secure it against oxidation in every part to which they are applied[1].
A fact so unprecedented is interesting to chemistry and is entitled to particular attention, as tending to enrich the science by the introduction of new ideas[2]. Confining myself in this place to the colours produced on metals by the action of fire, I do not hesitate to say that I think
- ↑ In order to give an idea of the efficacy of this preservative, it will be sufficient to quote the following experiment performed in Paris two years ago. I took two steel plates of the same quality and polish. I coloured one of them by the ordinary process, and exposed both in the open air to all the vicissitudes of a rainy autumn. At the end of a month the uncoloured plate was all rusted; the other had lost a little of its colour but was free from rust.
- ↑ If it were allowed me to offer an hypothesis relative to this novel state, I should say that the electro-negative elements disposed in thin layers on the surface of the metals are at too great a distance from the molecules of these
