produced by Electro-chemical Agency and hy 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. Sulphur 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 Monte Rosa, in the Alps; and on again visiting it in 1821, it was 

 found neither rusted nor corroded, but had mei'ely acquired a tarnish of 

 the colour of bronze, owing to the extreme dryness of the air at that ele- 

 vation. 



