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 phenomenon 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 
hot 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 phe- 
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+. 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 
_ * Tn 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 amonth the uncoloured plate was all rusted ; 
the other had lost a little of its colour but was free from rust. 
__ + Ifit were allowed me to offer an hypothesis relative to this novel state, I 
ould say that the electro-negative elements disposed in thin layers on the 
rface of the metals are at too great a distance from the molecules of these 
