134 COLOURS OF HEATED METALS. 



have others, so as to change the reflected colour, without any 

 alteration in the transmitted colour. 



Hitherto there is no case known, that allows us any foun- 

 dation to consider a body that is perfectly transparent, or 

 even a httle turbid, as composed of parcels of a given thick- - 

 ness, and kept at a necessary distance, in order to produce 

 a colour dependant on the thickness of its elementary 

 parts. 

 Colours of pel- Lastly the colours of pellicles are in certain cases variable 

 the incliiiatioiri ^Y *^'^ inclination of the light and of the eye, and some- 

 oftherays,and times too by the influence of the mediums with which they 

 medium.^ are in contact. Nothing similar to this takes place in the 



colours proper to the particles of bodies; for these are fixed 

 and permanent in whatever direction we look at them, and 

 are equally unchangeable by immersion in a dilferent fluid 

 medium of less or greater density. 

 Permanent co- These characteristic differences I conceive are sufficient 

 haveadrff" ^nl ^^ authorize the opinion, that the colours of substances in 

 capse. masses have not the same origin as those of thin pellicles ; 



a conclusion as important with respect to its object, as to 

 the difference of opinion that still subsists on it amoi\g the 

 learned *. 



I shall conclude with some observations on two curious 



kinds of phenomena, analogous to the subject, which I think 



I have sufficient grounds to explain in a manner different 



from that'generally admitted. 



Colours of ' The first relates chiefly to the colours of annealed steel. 



heated steel, Newton has ranked these amone those that depend on co- 

 referrid to the , , . „ . , . . , . . 



same cause by loured rings; not irom a particular exammation, but simply 



Newton J as a consequence of the system he had formed, supposing 



that the magniturle of the metallic particles must have beea 

 altered by the action of the fire. He did not consider whe- 

 ther thete were any jother causes, between which a choice was 

 to be made, 

 to oxigenation lyiore modern philosophers on the contrary have ascribed 

 dlxns' "^°' tl)ese colours without any hesitation to a different degree of 



* See among others in the 2d edition of Berth oil et's Elements of Dye- 

 ing, and the 2d edition of Haiiy's Treatise on Natural Philosophy, the 

 discussions and opp<5'sife bpinions of theffe celebrated authors on this, 

 question, > - 



oxidation, 



