58 Mr. Warington on coloured Films, S^c. 



While arguing at page 110 against the probability of the 

 principle of oxidation. Prof. Nobiii observes that the violet 

 tint on steel. " does not, perhaps, consist solely of oxygen, as 

 it does when the metals are pure. Steel is a carburet of iron, 

 and the oxygen of the air on being precipitated on this com- 

 pound, becoming combined with the carbon, in some manner 

 or other might form the layer in question." Now this is a 

 most extraordinary statement. After arguing so long and so 

 energetically against all thoughts of the oxygen uniting with 

 the metallic surface, it is here hinted that, in some manner or 

 other, the oxygen will take the carbon away from the iron, 

 with which it was in combination, unite with it, and form, not 

 a film of oxygen, but of carbon and that element, I suppose 

 carbonic oxide or carbonic acid ; and yet the theory advanced 

 as an explanation for these phsenomena, supposes that the 

 electro-negative elements disposed in thin layers are at too 

 great a distance from the molecules of the metal to enter into 

 combination with them. These statements surely are contra- 

 dictory. Allusion is made to the iridescent surface of the 

 specular iron ore, and they are successfully imitated by electro- 

 chemical means as given in the first part of this paper, but it 

 must be borne in mind, that by this means lead in a high stage 

 of oxidation is produced, and not a film or films of oxygen 

 alone. 



To sum up the whole of this subject in a few words, then, 

 it appears : 1st, that the appearances called electro-chemical 

 are not films of oxygen and acid, but lead in a high stage of 

 oxidation thrown down on the surface of the metal by means 

 of a voltaic combination acting through a medium formed by 

 a solution of acetate of lead ; 2ndly, that these colours owe 

 their varied tints to the varj'ing thickness of the precipitated 

 film, and that the light is reflected through them from the 

 polished metallic surface below; 3rdly, that the colours pro- 

 duced on the surface of metals by the application of heat are 

 owing to the formation of thin films of oxide of the metal in 

 consequence of exposure to the air during the process ; that 

 this does not involve the necessity of any one oxide being al- 

 ways formed, as this must vary according to the affinity of 

 the metal used for oxygen, under the influence of a raised 

 temperature; 4thly, that the opacity of the metal is not in 

 the slightest degree an argument against the transparency of 

 the oxide, as we have both in nature and art numerous cases 

 which place this question beyond a doubt ; 5thly, that we can 

 produce analogous appearances by substituting other elements 

 for oxygen, such as iodine, chlorine, bromine, sulphur, phos- 

 phorus, carbon, &c. 



