and partictdarli/ the Oxidation of Iron. i'3I 



tion is exposed to the air, the iron becomes more oxidated, 

 and as it then requires a greater quantity of muriatic acid 

 to dissolve it, it is partly precipitated. No nitrate, however, 

 exists in which the oxide is white. We can only obtain green 

 and red nitrates of iron 3 the first is formed by taking five 

 parts of the acid, and the second by employing twelve or 

 fifteen parts of acid. If the acid was more concentrated, a 

 portion of the red oxide would be precipitated, and very 

 little of it would be found in solution, if it contained from 

 thirty-six to forty parts. This precipitation would not take 

 place, unless the red oxide was very little divided : it is to 

 this cause that we ought to attribute ihe inaction of the nitric 

 acid upon colcothar, as well as the little action which sul- 

 phuric acid itself has upon this substance. What proves 

 this is the. property which these two acids have of easily dis- 

 solving the red oxide in the gelatinous state, or recently pre- 

 cipitated from the nitrate or muriate by alkalis. 



We find in the other acids the same mode of action as in. 

 the sulphuric acid ; but the generality of salts which result 

 from them, being insoluble, are rather obtained by means 

 of double decompositions than directly. I shall not here 

 take notice of all the saline bodies, on account of the little 

 interest they have hitherto excited in the arts and sciences ; 

 I shall only examine the two most important, — 'the gallatcs, 

 whic^i sei-ve asthe bases of the black dye ; and the prussiatcs, 

 the use of which has so much increased for these thirty years 

 past. 



We know that the gallic acid attacks iron even at the 

 temperature of the atmosphere; that it dissolves it with the 

 disengagement of that proportion of hydrogen gas necessary 

 when water is decomposed ; that this solution, at that tmie 

 colourless, becomes blue very soon on its exposure to the 

 air; and that then it begins to be thick, and passes to a 

 blackish gray. All these phcenon)ena have been described 

 with much care by M. Proust, but to this moment the whole 

 have not received a satisfactory explanation. I discovered the 

 cause of their difference in the thre^ oxides which iron is 

 susceptible of forming. Thus, by pouring gallic acid into 

 acidulated sulphate of white iron, no precipitate is obtained ; 

 P 4 with 



