108 M.NOBILI ON COLOURS, AND ON A NEW CHROMATIC SCALE 
necessarily different from that produced by the ordinary thin plates, 
which are so transparent as to arrest no species of rays whatsoever. 
The blond, as already observed, contains a tinge of green which is not 
found in the beautiful yellow of gold. If we leave out this green, by 
supposing it absorbed in the process of reflexion, the result will be a tint 
very closely resembling, if not exactly equal to, that of gold. 
The red of copper requires a similar reduction. The colour nearest 
to it is the tawny of No. 7. But this contains a cast of violet, which 
is not in the copper, and the removal of it will make the resemblance, 
if not complete, certainly much less imperfect. 
It is not my purpose in this place to enter further into the question, 
‘or to investigate the causes to which it is owing, that coloured bodies 
absorb certain rays more rapidly than others. The fact itself is proved, 
and it is unnecessary to go further for the attainment of our object, 
which was to discover the cause of the great difference between me- 
tallic colours and those of thin plates. 
Colours developed on Metals by the Action of Fire. 
The prismatic colours produced on steel and copper by the action of 
heat are universally known. Analogous colours are also exhibited by 
tin, bismuth, lead, &c., when they are in a state of fusion. 
As to these colours, the most generally received opinion is, that they 
depend on a principle of oxidation. Berzelius calls the metallic layer 
which is thus coloured a suboxide*. 
I have always entertained some doubts as to the correctness of this 
explanation; because each degree of oxidation has a colour peculiar to 
itself, and in no way related to that variety of tints of which we speak. 
I was also struck by the well-known practice of giving steel a violet 
colour in order to secure it from rust. We know that this colour is 
produced by means of fire, in the process of giving steel a particu- 
lar temper,—a temper which is called violet, because it is produced 
simultaneously with the colour. Were this tint, as it is presumed to be, 
the effect of oxidation, it would, in my opinion, instead of preventing, 
serve only to accelerate oxidation. A very high degree of polish, I 
allow, will keep off rust for a long time, but cannot stop it when once 
the action has commenced. 
. * Some persons fancy that the phenomenon arises from the mere displacing 
of the parts, and thus exclude the intervention of any other substance. Ac- 
cording to this notion it is but the metal dividing itself into laminz of different 
degrees of thickness, and thus becoming capable of producing the different — 
colours. Such an opinion, however, is opposed to a positive fact already de- 
monstrated; I mean the fact of their opacity being in all cases too great to 
admit of their furnishing laminz sufficiently transparent to produce the colours 
in question. 
