OF ARTS AND SCIENCES. 203 



pared for analysis in this way, because a mixture of substances is the 

 result to be expected from the incomplete reaction. Again, the partial 

 oxidation of hydrated chromic oxide may be effected by exposing it for 

 a long time to a temperature above 200°, but below redness, with free 

 access of air.* 



The most precise statement on this subject has been made by Krii- 

 ger, t whose main purpose was to prove that the glowing of ignited 

 chromic oxide was due to the sudden escape of oxygen absorbed at a 

 lower temperature, but who incidentally maintains that this absorption 

 of oxygen gives rise to a definite oxide of chromium whose formula is 

 CrOj. We think to be able to show, first, that no definite compound 

 whatever is formed during this imperfect oxidation ; secondly, that the 

 substance which really results from the prolonged heating contains 

 chromic acid, and is an indeterminate approximation to the body whose 

 formula is CrjOs CrOs. It is obviously impossible to expose all parts 

 of a substance in powder, like the hydrated chromic oxide, to the uni- 

 form action of the same quantity of air at the same temperature for 

 the same time, and the limits of temperature between which the de- 

 sired absorption of oxygen will take place most readily are not very 

 clearly defined. Under these circumstances we should expect to ob- 

 tain, not a definite compound, but a mixture, and the figures of Kriiger's 

 own analysis fully confirm this expectation. Kriiger found 63.70 per 

 cent of chromium and 36.30 per cent of oxygen in the body which 

 he analyzed ; the supposed oxide of chromium, CrO^ , would contain 

 62.12 per cent of chromium, giving a discrepancy of 1.58 per cent 

 between the chromium of Kriiger's substance and the chromium of 

 the imaginary CrO^. This does not seem an inadmissible error, till we 



* The partial conversion of chromic oxide into chromic acid by gentle roasting 

 seems to have been applied in the arts years ago. Cooley, in his Cyclopaedia of 

 Practical Receipts, (London, 1845, p. 263,) describes Charles Watt's process of 

 preparing chromic acid from the oxide of chromium precipitated hy lime from the 

 residual liquor of the process of bleaching with chromic acid. The precipitate was 

 heated evenly in a thin layer on a flat iron plate, with frequent stirring, till the mass 

 assumed a yellow color. If too much heat was employed, the product of this op- 

 eration was easily decomposed, assuming a green color. This process was appar- 

 ently a true conversion of chi-omic oxide into chromic acid by roasting, and should 

 not be confounded with the method of preparing chromate of lime described by 

 Jacquelain (Ann. de Ch. et de Phys., [3.] XXI. 478), in which a mixture of lim^ 

 and chrome-iron ore was heated to redness in a reverberatory furnace. 



t Jour. pr. Ch., XXXII. 383, (1844,) and Pogg. Ann., LXI. pp. 219, 406. 



