210 PROCEEDINGS OF THE AMERICAN ACADEMY 



acid by paper, and by the digestion of chromic oxide with chromic 

 acid. 



Thomson* prepared a brown precipitate, which he called brown 

 oxide of chromium, by passing a stream of sulphurous acid through a 

 solution of chromate or bichromate of potash. We have found, as did 

 Berthier,t that no precipitate whatever is produced by passing sul- 

 phurous acid through bichromate of potash ; but this is a point of no 

 importance in this connection. Thomson washed the precipitate which 

 he had obtained for two months, and noticed the steady abstraction of 

 chromic acid from the precipitate. He finally analyzed the washed 

 oxide (as he called it) and made it to be a very basic chromate cor- 

 responding to the formula (Cr203)6 CrOs- His opinion of the effect 

 produced by the ignition of tlie nitrate of chromic oxide we have already 

 cited. Under the head of chromate of chromium he remarks that, when 

 chromic oxide is dissolved in chromic acid and the solution evaporated, 

 there remains a substance quite similar in appearance to the brown 

 oxide of chromium. Again, he observed the precipitate produced by 

 mixing chromate of potash with sesquichloride of chromium, and says 

 of it that it is evidently composed of chromic acid and the green oxide 

 of chromium. Guided by the analogy of chromium and iron, he pre- 

 pared a chromate of iron by mixing chromate of potash with sesqui- 

 chloride of iron ; an analysis of the edulcorated brown precipitate led 

 him to the fonnula (Fe203)5 CrOg, and in the filtrate and wash-water 

 he thought he found another less basic chromate, corresponding to the 

 formula (Fe203)5 (Cr03)2.5. On the whole, Thomson seems to have 

 believed in the existence of a brown oxide of chx'omium, intermediate 

 between chromic oxide and chromic acid ; but every one of his experi- 

 ments and analyses points directly to the conclusion, that the supposed 

 oxide is in reality a chromate of chromic oxide, or rather in most cases 

 an indeterminate mixture of chromic oxide and chromic acid. 



We come now to the researches of Maus,J contemporaneous with 

 those of Thomson, but much more conclusive upon the disputed point 

 as to the existence of a distinct oxide of chromium answering to the 

 formula Cr02. Maus mixed an aqueous solution of sesquichloride of 

 chromium with chromate of potash, and digested the washed precipitate 



* Phil. Trans., 1827, Part I. p. 186. 



t Ann. der Ch. u. Pharm., XL VI. 185 (1843). 



t Pogg. Ann., IX. 127 (1827). 



