OF ARTS AND SCIENCES. 205 



cipitate used in analysis/, we obtained without difficulty the red chro- 

 mate of chloride of chromium by heating it with chloride of sodium and 

 strong sulphuric acid. We shall have occasion to cite below a similar 

 experiment upon an anhydrous mixture of chromic oxide and chromic 

 acid, obtained by gently igniting the nitrate of chromic oxide, in which 

 the chromate of the chloride of chromium was very readily obtained. 

 Relying on the yellow color imparted by the substance under examina- 

 tion to water and to chloride of ammonium, and on the ready exhibition 

 of perchromic acid by means of peroxide of hydrogen, and explaining 

 Kriiger's failure to obtain chlorochromic acid by the fact that only a 

 very small amount of chromic acid proportionally exists in the mixture, 

 we conclude that, by long heating in the air, a part of the chromic oxide 

 is converted into chromic acid, which instantly combines with other 

 chromic oxide, and that the end of this process, seldom if ever attained, 

 is the conversion of the whole mass into the chromate of chromic oxide, 

 CrjOs CrOs. 



5. Many chemists, among whom may be mentioned Vauquelin, Berze- 

 lius, Dobereiner, and Thomson, have tried the experiment of gently ignit- 

 ing the nitrate of chromic oxide, or, what amounts to the same thing, of 

 evaporating to dryness nitric acid in contact with metallic chromium or 

 hydrated chromic oxide, and moderately heating the residue. Some, 

 like Berzelius,* have thought that they obtained in this way a definite 

 oxide of chromium, intermediate between chromic oxide and chromic 

 acid, and answering to the formula CrOa ; others, like Vauquelin,t have 

 imagined that they obtained chromic acid by repeated evaporation of 

 nitric acid with chromium, or have believed, with Thomson J and Go- 

 don,! that a great part of the green oxide was converted into chromic 

 acid ; while others still have maintained, with Dobereiner, || that the 

 chromate of chromic oxide was formed by the decomposition of the 

 nitrate. 



In order to a clear knowledge of the effect of evaporating nitric acid 

 with chromic oxide, it is necessary in the first place to answer qualita- 



* Thomson's Ann. Phil., III. 104, (1814,) and Schweigger's J. fiir Ch. u. Phys., 

 XXII. 56. 



t Ann. de Chim., XXV. 201 (1798). 



t Phil. Trans., 1827, Part I. p. 206. 



§ Rapport par MM. Berthollet et Vauquelin, Ann. de Ch., LIII. 224. 



II Schweigger's Jour, fiir Ch. u. Phys., XXIT. 482 (1818). 



