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 metallic 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[1].
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 particular 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 phænomenon arises from the mere displacing of the parts, and thus exclude the intervention of any other substance. According to this notion it is but the metal dividing itself into laminæ 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 demonstrated; I mean the fact of their opacity being in all cases too great to admit of their furnishing laminae sufficiently transparent to produce the colours in question.
