203 PROCEEDINGS OF THE AMERICAN ACADEMY 



lively the question, Is chromic acid formed during the process ? A 

 mixture of nitric acid and hydrated chromic oxide was gently evapo- 

 rated, first on a water-bath, then upon a sand-bath, and before the free 

 nitric acid was completely driven off, a portion of the moist mass was 

 treated with a solution of caustic potash ; it partially dissolved with a 

 yellow color, and the solution gave the distinct blue of perchromic acid 

 with peroxide of hydrogen. The rest of the evaporated residue was 

 heated till it looked like a perfectly dry powder, but still retained the 

 smell of nitric acid. In this condition a portion was treated with water, 

 in which it slowly and partially dissolved, yielding a brownish yellow 

 solution, which readily gave the blue of perchromic acid. A half of 

 the remaining residue from the original evaporation was strongly heated 

 in a porcelain dish on a sand-bath, and then digested with water during 

 four hours ; the solution so obtained was decidedly yellow, and gave 

 easily the blue of perchromic acid. The other half of the residue was 

 heated on platinum foil, till the platinum was of a dull red color in a 

 darkened room, and was then soaked in water for thirty-six hours ; the 

 supernatant liquid was of a pale yellow color, and gave a faint blue with 

 peroxide of hydrogen. The same substance treated with caustic potash 

 gave a yellow solution. 



With this qualitative evidence of the formation of an abundance of 

 chromic acid at a moderate temperature, which is not wholly destroyed 

 even at incipient redness, we proceeded to the quantitative determina- 

 tion of the amount of chromic acid formed at the different stages of the 

 process. A quantity of pure hydrated chromic oxide was treated in a 

 porcelain dish with an excess of pure nitric acid, and evaporated on a 

 water-bath nearly to dryness ; those parts of the substance which dried 

 completely were moistened with nitric acid and again dried. The nitric 

 acid could not be completely driven off on the water-bath ; though the 

 larger part of the mass seemed dry, yet portions of it were still moist 

 when the process was stopped, and it was far from being homogeneous. 

 The brownish black substance imparted a strong yellow tinge to water, 

 and dissolved with difficulty in dilute nitric acid, giving a brownish 

 yellow solution. A portion of it, analyzed by Rose's method, gave 

 CT2O3 = 64.85 per cent. 

 CrOs = 31.51 

 NO5 and HO = 3.64 " (by difference). 

 It was now necessary to prepare for analysis from the evaporated resi- 

 due a series of homogeneous substances, each of which had been exposed 



