58 
Mr, Warington 07i coloured Films^ ^c. 
While arguing at page 110 against the probability of the 
principle of oxidation, Prof. Nobili 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 pheenomena, 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 varying 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 notin 
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. 
