OXIDATION OF IRON. 107 



gen conducted to the top of the experimental tube ; and it 

 was then easy to introduce into them traces of moisture, 

 carbonic acid, and ammonia. After a period of four months 

 the blades of iron so exposed gave the following results : — 



Dry oxygen No oxidation. 



D t> ot frpn f ^ n three experiments, only one blade 



^ y° \ slightly oxidized. 



Dry carbonic acid No oxidation. 



( Slight appearance of a white precipitate 

 on the surface of the iron — found to 



Damp carbonic acid ■{ be carbonate of iron ; two only out 



of six experiments did not give this 



^ result. 



Dry carbonic acid and oxygen No oxidation. 



' Oxidation most rapid, a few hours being 

 sufficient ; the blade assumed a dark 

 green colour, which then turned 

 brown-ochre. 



Dry oxygen and ammonia No oxidation. 



• Damp oxygen and ammonia No oxidation. 



Damp oxygen and carbonic acid 



The above results prove that, under the conditions de- 

 scribed, pure and dry oxygen does not determine the oxida- 

 tion of iron — that moist oxygen has only a feeble action — 

 dry or moist pure carbonic acid has no action — but that 

 oxygen containing traces of carbonic acid acts most rapidly 

 on iron, giving rise to protoxide of iron, then to carbonate 

 of the same oxide, and lastly to a mixture of saline oxide 

 and hydrate of sesquioxjde. 



As these facts tend to show that carbonic acid is the 

 agent which determines the oxidation of iron, I am justi- 

 fied in assuming that it is the presence of carbonic acid in 

 the atmosphere, and not its oxygen or aqueous vapour, 

 which determines the oxidation of iron in common air*. 

 Although this statement may be objected to at first sight, 

 on the ground of the small amount of carbonic-acid gas 

 existing in the atmosphere, still we must bear in mind 

 that a piece of iron, when exposed to its influences, comes 



* These results prove the statement of Bonsdorff (see Gmelin, vol. v. 

 p. 185), that carbonic acid has no action on iron, is incorrect. 



