226 Oil tJie Oxidation of Metals in general, 



that there was also a yellow oxide, hecaase ferruginous salts 

 present themselves sometimes under this colour. Reasoning 

 upon the hypothesis that all the metallic salts were always 

 ot the same shade as their oxides, they were necessarily led 

 to adopt that idea; but it is now very well known that this 

 method of viewing the subject is often erroneous, and that 

 a salt is often white or red although its oxide is red or blue. 

 Another salt, consequently, might be yellow, and have a 

 red oxide for its base : this is exactly the case with all the 

 yellow sails of iron. Besides the green and red oxide, how- 

 ever^ there exists another oxide of iron which performs a 

 very important part in the ferruginous saline combinations j 

 and as, on the one hand, the formation of several produc- 

 tions manufactured in the workshops, and, on the other hand, 

 as the explanation of several phaenomena, which often present 

 themselves in the arts, depend upon the existence of this ox- 

 ide, I think it my duty to enter into some details respecting it. 



It is formed when iron is treated with most of the acids; 

 and we obtain it by decomposing by means of potash, soda, 

 and ammonia, the various salts so formed. Above all, by 

 means of a solution of iron in sulphuric acid we may easily 

 demonstrate ils existence. If in this recent solution we pour 

 some alkali, a white precipitate is formed, which becomes 

 speedily green at the surface and soon passes to a red co- 

 lour *. These changes of colour, which take place in the 

 whole mass if it is agitated, are evidently produced by an 

 absorption of oxygen ; because, in repeating the experi- 

 ment in a fiask, the air which it contains sensibly diminishes 

 in volume, and sopn extinguishes a taper plunged into it, 

 when the white oxide has become green, and with greater 

 reason when it has passed to the red. 



It is this white oxide, hypersaturated with sulphuric acid, 

 which forms, in a great measure, the sulphate of iron used 



• I poured seme hyneroxidated sulphate of iron into a great eicess of 

 caustic potash, and I always observed a white precipitate. 1 even boiled the 

 mixture in a retort the neck of which was plunged into water; the oxide-be- 

 came green, and was even rtd at the surface ; but the lower part was always 

 perceptibly white, although it contained no sulphuric acid. Of this I con- 

 vinced myself by w ashing it, dissolving it in the muriatic acid, and by adding 

 nitrate oi barytt^s to it. 



in 



