268 OXIDATIONS OF IRON. 



White oxide. This reasoning led me to examine not only the red and 

 green oxides, but at the same time the white oxide lately 

 announced by Mr. Thenard*, of which I had beforehand 

 some doubtsf. I am sorry to call in question the labours of 

 men whom I greatly esteem and respect, but I conceive I am 

 seconding their views, if my observations be just. 



Of the While Oxide. 



Its characters. The characteristics assigned to this oxide by Mr. The- 

 nard are, 1st. that it becomes green by exposure to the air; 

 2d. that it is changed to green or yellow by oxigenized mu- 

 riatic acid ; 3d. that when the precipitate is made in a phial, 

 and care is taken to stop it close, an absorption is observed 

 on shaking it to convert the white oxide to green; which 

 proves, that part of the oxigen of the air in the phial com- 

 bines with the white oxide, and changes its colour. I shall 

 take the liberty of examining these facts. 



Change of co- From the experiments of Messrs. Fourcroy, Proust, and 



lourinprecipi- Berthollet, Mr. Thenard, and all the chemists of the pre- 

 tates no proof ' , r 



of a different sent day, are of opinion, that the change of colour in pre- 

 •tose of oxigen cipitates does not in general indicate a different degree of 

 oxygenation. The white is that which very frequently con- 

 ceals the true colour of oxides in almost all metals, that are 

 said to be susceptible of oxigenation, as tin, mercury, cop- 

 per, silver, lead, bismuth, and probably manganese. This 

 depends on the quantity of acid the precipitates retain, 



* See our Journal, Vol. XIV, p. 224. 



•f Before undertaking my present inquiry I could not avoid doubting,, 

 that a few hundredth parts of oxigen could modify the colour of metallic 

 oxides so far as to change them from white to green, black, &c. All tha, 

 facts witli which I am acquainted militate against this property of oxi' 

 gen; consequently, I was suspicious of every precipitate, the colour of 

 which varied much from that of the oxides of the same metal obtained 

 by calcination. If white, 1 argued, be the result of the combination of 

 iron with a few hundredths of oxigen, why does not this colour present it- 

 self in the course of the calcination of iron ? and why in the calcination of 

 manganese, copper, and bismuth, do we not perceive the same phenome- 

 non ? Besides, the only well established vvhite oxides are those of anti- 

 mony, zin, and arsenic ; and from the moment these become white they 

 Rcver change their colour, even from a considerable over dose of oxigen. 



they 



