OF ARTS AND SCIENCES. 197 



the precipitate filtered off, washed, strongly ignited, and the chromic 

 acid calculated from the pure chromic oxide which remained. To the 

 filtrate from this first precipitate ammonia in excess was added, and the 

 washed precipitate, strongly ignited, was the chromic oxide contained 

 in the original substance. 



Analyses. 



g. h. 



CrgOs 33.06 per cent. 33.54 per cent. 



CrOs 32.03 " " 30.53 " " 



HO (by difference) 34.91 " " 35.93 " " 



It is quite evident from these analyses that the precipitate caused by 

 mixing chromate of potash with a neutral chrome salt cannot contain 

 more than one equivalent of chromic acid in combination with its chro- 

 mic oxide, since in the above determinations the chromic acid is nearer 

 one equivalent than two with reference to the chromic oxide found, in 

 spite of the fact that the considerable amount of chromic acid which, 

 combined with potash, has adhered mechanically to the precipitate, is 

 thrown down by the nitrate of the suboxide of mercury, together with 

 the chromic acid which was originally united to the chromic oxide. 

 But the fact that the substance under examination cannot be purified 

 by washing without altering its constitution, renders it impossible to 

 ascertain the exact composition of the body by the methods of analysis 

 heretofore used, or by any similar methods ; for this purpose the pro- 

 cess used by Vogel * in analyzing this same substance, obtained by him 

 from a different source and miscalled CrOg, is admirably adapted. The 

 precipitate examined by this method was not washed at all, but was 

 simply dried by pressure between folds of filter-paper and exposure 

 to the air. A weighed portion was placed in the bulb of a reduction- 

 tube, with which a weighed chloi'ide of calcium tube was connected ; a 

 current of dry air was then drawn through the apparatus, and the re- 

 duction-tube was cautiously heated till all the water of the precipitate 

 had been absorbed by the weighed chloride of calcium tube, the heat 

 finally rising to dull redness. The salts with which the precipitate 

 under examination was contaminated were sulphate of potash, bichro- 

 mate of potash, and a little chromate of potash, and since the solutions 

 from which the precipitate was prepared were concentrated, it was to be 



* Jour. pr. Ch., LXXVII. 484 (1859). 



