PRODUCED BY ELECTRO-CHEMICAL ACTION. 109 



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 sud- 

 denly transparent in consequence of their union with a small quantity 

 of oxygen ? The hypothesis far exceeds the bounds of probability, and 

 the phaenomenon 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 ap- 

 paratus 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 ox- 

 idized, 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 cer- 

 tain 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 phae- 

 nomenon of the coloured rings in all its beauty ; 3rd, instead of ox- 

 idizing the metal, these electro-negative elements contribute to secure 

 it against oxidation in every part to which they are applied *. 



A fact so unprecedented is interesting to chemistry and is entitled to 

 particular attention, as tending to enrich the science by the introduc- 

 tion of new ideas f. Confining myself in this place to the colours pro- 

 duced 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. 

 1 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. 



t If it were allowed me to ofler 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 gi-eat a distance from the molecules of these 



