108 M. NDBIH 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 unnecessaiy 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 colojurs, 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, 

 «erve 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 phaenomenon 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 laminae 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 laminae sufficiently transparent to produce the colours 

 in question. 



