204 PROCEEDINGS OF THE AMERICAN ACADEMY 



observe that the whole difference between the chromium in chromic 

 oxide (68.62 per cent) with which Kriiger began, and the chromium 

 in the supposed higher oxide CrOa (62.12 per cent), is only 6.50 per 

 cent, and that his error of 1.58 per cent in the amount of chromium is 

 therefore nearly twenty-five per cent of the whole difference between 

 the bodies CrgOa and CrO.2. With so fatal a discrepancy between the 

 actual and the theoretical figures, it is of course impossible to main- 

 tain that a definite oxide is obtained by such an indefinite process. 

 We pass to the second point, the existence of chromic acid in imper- 

 fectly oxidized chromic oxide. Kriiger thought to disprove the pres- 

 ence of chromic acid by heating the substance with sulphuric acid and 

 common salt ; no chromate of chloride of chromium being visible, he 

 inferred the absence of chromic acid. This test is at best but an un- 

 satisfactory one when applied to a very insoluble substance containing 

 only a small proportion of chromic acid, and there seems to be no 

 reason for trusting to a coarse reaction in a difiicult case when very 

 delicate tests are at hand. By heating hydrated chromic oxide for five 

 hours to a temperature varying between 200° and 210° we obtained a 

 brownish black powder, which dissolved with great difiiculty in dilute 

 acids, communicating a dark yellow color to the liquid. Digested with 

 water, the powder yielded a partial solution of a bright yellow color, 

 and this solution gave a very marked reaction for perchromic acid with 

 the solution of peroxide of hydrogen.* When quickly boiled with an 

 aqueous solution of chloride of ammonium, the filtered solution was 

 bright yellow, and gave the reaction of perchromic acid with peroxide 

 of hydrogen. Would it not be difiicult to explain this effect of aqueous 

 chloride of ammonium on the brown powder on the supposition that its 

 real composition was represented by the formula CrOa? Kriiger en- 

 deavored to strengthen his position with regard to the absence of chro- 

 mic acid, by heating with common salt and sulphuric acid a precipitate 

 prepared by mixing bichromate of potash, sulphate of chromic oxide, 

 and ammonia, and finding as before no chromate of chloride of chro- 

 mium ; he used the precipitate in one experiment air-di'ied, in another, 

 dried at 110°. This precipitate was probably a mixture of chromic 

 oxide with the chromate of chromic oxide, and, unless the last ingre- 

 dient was present in very small proportion, it should have readily pro- 

 duced chromate of chloride of chromium. From a portion of the pre- 



* Storer, Proc. Amer. Acad., IV. 138 ; Jour. pr. Ch., LXXX. 44. 



