196 PROCEEDINGS OF THE AMERICAN ACADEMY 



sis/). Tlie process of analysis was the same described above, and the 

 results were as follows : — 



Analyses, 

 d. 

 CrsOs 38.85 per cent. 



CrOa 25.91 " « 



HO (by diff.) 35.24 " « 



This set of analyses coiToborates the first series in every respect, and 

 the ratio of the chromic oxide to the chromic acid of the substance d is 

 almost precisely the ratio 3 : 2 of the chromic oxide to the chromic acid 

 in the chromate of chromium CrjOs CrOa- We again see that there is 

 no definite limit to the removal of chromic acid by prolonged washing, 

 and that the substance may be readily made more basic than any imag- 

 inable definite compound of chromic oxide and chromic acid would be. 



In these analyses the precipitates had been somewhat washed before 

 any analysis was made ; it remained to analyze the substance precifji- 

 tated with so much of the adhering filtrate as could not be removed by 

 pressure between folds of filter-paper without washing. One precipi- 

 tate (analysis g) was prepared by mixing a solution of one equivalent 

 of chrome alum with eight equivalents of chromate of potash in solu- 

 tion, and a second by mixing a solution of one equivalent of chrome 

 alum with sixteen equivalents of chromate of potash in solution (analy- 

 sis h). In both cases the solutions used were concentrated and the 

 precipitates air-dried. The method of analysis which we had hereto- 

 fore employed was open to one objection, — a trace of chromic oxide 

 was liable to be dissolved in the excess of ammonia, to be again sep- 

 arated when the filtrate and wash-waters from the chromic oxide pre- 

 cipitate were concentrated by evaporation, and the process would obvi- 

 ously be altogether inadmissible in any case where the sulphates of the 

 mother-liquor had not been removed by washing before submitting the 

 precipitate to analysis, since sulphate of lead would be formed and 

 vitiate the determination of the chromic acid. We therefore resorted 

 to Rose's method of separating chromic oxide from chromic acid, by 

 means of the nitrate of the suboxide of mercury.* The solution of the 

 substance to be analyzed in dilute nitric acid was nearly neutralized 

 with carbonate of potash, and when the carbonic acid had been allowed 

 sufficient time to escape, nitrate of the suboxide of mercury was added, 



* Handbuch der Analytischen Chemie, (Braunschweig, 1851,) II. 379. 



