208 PROCEEDINGS OP THE AMERICAN ACADEMY 



contact with the air, a substance which, as we have seen, is also an in- 

 determinate mixture of chromic oxide and chromic acid, containing, how- 

 ever, very much less chromic acid than the mixture just examined. 

 Substance No. 5 of the preceding series, although anhydrous, yields 

 chlorochromic acid with salt and sulphuric acid with the greatest facil- 

 ity, another indication that the true explanation of Kriiger's not obtain- 

 ing chlorochromic acid from his heated chromic oxide is to be found in 

 the fact that it contains too small an amount of chromic acid to be ex- 

 hibited by that somewhat coarse reaction. 



6. Many distinguished chemists have observed the reactions and 

 studied several of the substances which we have here described, and 

 no treatment of the subject can be complete which does not embrace 

 an abstract of their labors. We shall therefore relate as concisely as 

 possible the history of the substance which has been variously called 

 brown oxide of chromium, or chi'oraate of chromic oxide, and review 

 the discussion as to its composition which has heretofore been brought 

 to no satisfactory conclusion. 



Vauquelin,* the discoverer of chromium, obtained what he thought to 

 be a brownish-red oxide of chromium by several methods, the details 

 of which it is unnecessary to describe ; it will be enough to observe, 

 that neither of his methods gave him a definite compound ; they all 

 gave rise to indeterminate mixtui-es of the bodies now known as 

 chromic oxide and chromic acid. He proved that nitric acid, whether 

 cold or boiling, cannot oxidize chromic oxide, but that the calcination 

 of the nitrate of chromic oxide produced a substance whose aqueous 

 solution was red ; ammonia precipitated from this solution the green 

 oxide, and the separated filtrate was yellow. His experiments led 

 Vauquelin to the conclusion, that there are two kinds of oxide of 

 chromium, which differ only in the quantity of oxygen they contain. 



Two errors, into which the reader of Vauquelin's papers might 

 easily fall, demand notice. It might be inferred from some of his 

 statements that the chromate of ammonia was a body readily decom- 

 posed by simple boiling with separation of the so-called brown oxide 

 of chromium. We have found the chromate of ammonia to be a body 

 possessing much greater stability than has been usually attributed to 

 it ; its aqueous solution will resist, without change, prolonged boiling, 



* Ann. de Ch., LXX. pp. 85, 86 (1809). 



