56 Mr. Warington on the coloured Films 



laj'er which is thus coloured a suboxide." In a foot-note at 

 p. 110, we read, " Berzelius was more sensible of the diffi- 

 culty," (of accounting for these coloured films) " perhaps, 

 than any one else : but would not an open avowal have been 

 better than the attempt to evade it by the adoption of the 

 terra suboxide, which is quite as vague and undefined as the 

 principle of oxidation, for which it was offered as a substitute?" 

 This is rather strong language to be used against such an au- 

 thority as Berzelius, every one of whose statements is backed 

 by investigation, and brought by Prof. Nobili, who does not 

 adduce a single experiment in proof of his statements. But 

 to proceed : " 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 re- 

 lated 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." " Were this tint, as it is pre- 

 sumed to be, the effect of oxidation, it would, in my opinion, 

 instead of preventing, serve only to accelerate oxidation." 

 " But this is not all; the superficial colours of which we speak 

 are changeable, and belong evidendy to the same class as 

 those produced by thin plates. Now the pure metals are, 

 from their opacity, incapable of this species of coloration. 

 Can they acquire that capacity in their first degree of oxida- 

 tion by becoming suddenly transparent in consequence of their 

 union with a small quantity of oxygen ? The hypothesis far 

 exceeds the bounds of probability, and the phaenomenon re- 

 quires to be otherwise explained." Again, " Confining my- 

 self in this place to the colours produced on metals by the 

 action of fire, I do not hesitate to say, that I think their 

 origin now placed beyond the reach of doubt. It may be 

 safely laid down as a general proposition, that the oxygen of 

 the atmosphere produces them, not, as is supposed, by oxi- 

 dizing the surface of the metal, but by becoming fixed in the 

 form of a thin plate or film, similar to those of the electro- 

 chemical appearances." Professor Nobili then gives a detail 

 of the production of the colours by heat, and observes, that 

 as long as the colours are seen there is no oxidation, but that 

 when the metal loses its brilliancy and lustre it has become 

 oxidized, and that if removed from the heating medium be- 

 fore this effect takes place, the oxygen will cover the metal 

 and adhere as a varnish. 



In Berzelius's System of Chemistry, under the heads of the 

 various metals and the action of heat upon them, he states 

 distinctly, that copper, lead, and tin form protoxides, and that 

 palladium forms a suboxide; so that out of four metals which 



