produced by Electro-chemical Agency and by Heat, 57 
produce coloured films by heat, only one is stated to form a 
suboxide. The next point is, that each degree of oxidation 
produces colours peculiar to itself ; this is offered as a reason 
for doubting the principle of oxidation. No person will ima- 
gine that each tint on the surface of the metal is owing to a 
different oxide, but that it arises, as experiment proves, from 
the films of the oxide being of various thicknesses, and that 
the surface of the bright metal below reflects the light through 
the film, as is the case with the oxide of lead produced by 
the electro-chemical agency; and that the brighter the metallic 
surface the more vivid and brilliant are the colours produced. 
With respect to the preserving power of a film of oxide on 
the surface of the metal, it is well known ; and the principle 
on which it acts is the exclusion of moisture and carbonic 
acid from contact with that surface*. The next point is the 
capacity of metals to become transparent when united with 
oxygen, and here I think the evidence is satisfactory indeed. 
Copper, tin, antimony, titanium, zinc, and iron all occur in 
the mineral form transparent or translucent on the edges, and 
this in mass : of course, if reduced to thin films, such as are 
produced by heat, they would be perfectly so. 
The same beautiful display of colours on the surface of 
metals may be produced by iodine : the manner in which I 
have formed these is simply to place a very small piece of 
iodine on the centre of a disc of metal, — copper or silver suc- 
ceeds extremely well, — and cover the surface with a flat dish, 
so as to prevent the vapour of iodine from being dissipated : the 
circles of coloured films are very distinct and brilliant. The 
application of the thinnest or pale yellow of these films on 
silver by M. Daguerre, to the production of his pictures, is 
now well known. Sul})hur and the application of heat pro- 
duce similar effects. The process of oxidation by heat, and 
the disappearance of the metallic lustre, which Prof. Nobili 
attempts to use as an argument, are accounted for very simply 
thus ; that the application of the heat being continued, the 
film of oxide gradually becomes thicker,until at last it becomes 
opake ; but in the course of my experiments 1 have had these 
coloured films peel off from the surface of the metal with all 
their transparency and all their beautiful tints unimpaired. 
The same effect has taken place, by long-continued electro- 
chemical action, with the oxide of lead. 
* M. Zumstein, in August. 1820, fixed a polished iron cross on the 
summit of Montedlosa, in the Alps ; and on again visiting it in 1821 , it was 
found neither rusted nor corroded, but had merely acquired a tarnish of 
the colour of bronze, owing to the extreme dryness of the air at that ele- 
vation. 
