OF ARTS AND SCIENCES. 209 



concentration, and exposure for weeks to the direct rays of the sun. 

 It is true that repeated evaporation to dryness may partially decom- 

 pose it, and it is of course destroyed by ignition. 



Again, Vauquelin asserts that ammonia will precipitate the brown 

 oxide of chromium from an aqueous solution of chx'omic acid through 

 which sulphurous acid has been passed, and this assertion seems to 

 be confirmed by some statements, made without quoted authority in 

 Gmelin's Handbook (Cavendish Soc. Ed,, IV. 114), concerning salts 

 of the brown oxide of chromium obtained by dissolving this hydrated 

 oxide in acids, from which solutions it may be again precipitated by 

 ammonia. This is an important point in determining the real exist- 

 ence of such an oxide of chromium, and we have therefore made it the 

 subject of careful experiment. We have dissolved in dilute chlorhy- 

 dric and dilute nitric acids such precipitates as analysis had shown to 

 have very nearly the composition which is expressed by the formula 

 Cr02 (as, for instance, the precipitate of analysis d, and of the analysis 

 on page 201), and have added to the solutions ammonia of every 

 strength, from the strongest to the weakest, and have so obtained one 

 invariable result, viz. a precipitate of common green chromic oxide, 

 and a filtrate made yellow by chromate of ammonia. It is true that 

 the fresh precipitate has a dirty or brownish look, caused by its im- 

 pregnation with the yellow liquid in which it floats; and this- is the 

 most probable explanation of the opinion held by some previous 

 observers, that this precipitate was something more than ordinary 

 chromic oxide. 



The chief authority upon which the existence of salts of the brown 

 oxide of chromium is asserted, seems to be that of Brandenburg,* who 

 obtained solutions of substances which his own experiments, rightly 

 interpreted, prove to have been mixtures of chromic acid and salts of 

 chromic oxide, but which he thought were salts of an imaginary higher 

 oxide of chromium. There is no such thing as a salt of the brown 

 oxide of chromium, for the reason that there is no such base. 



It was the opinion of Dobereinerf that the substance produced by 

 the calcination of the nitrate of chromic oxide was a chromate of 

 chromic oxide and not an oxide of chromium, and he referred to the 

 same formula the substances formed by the reduction of chromic 



* Schweigger, Jour, fiir Ch. u. Phys., XIII. 287 - 289 and 299-304. 

 t Ibid., XXII. 482 (1818). 

 VOL. V. 27 



