OF ARTS AND SCIENCES. 199 



If the original precipitate was the compound whose formula is 

 Cr^Oa CrOs, the loss of oxygen by ignition would have been 0.0175 

 gram. ; the actual loss was 0.0183 gram., being within 0.0008 gram, 

 of the theoretical amount. A loss of eight tenths of a milligramme 

 in the water determination would of course account for this very 

 small excess of oxygen. 



A second analysis by this method of a precipitate containing a 

 smaller proportion of soluble salts corroborated the first determination 

 in every respect, and left no doubt that the true composition of the 

 precipitate produced in the aqueous solution of a neutral salt of 

 chromic oxide by the solution of chromate of potash is represented 

 by the formula CraOa CrOg. 



2. Schweitzer* observed that, when nitric oxide is passed through a 

 moderately dilute solution of bichromate of potash, a brown precipitate 

 separates after some time, especially if the liquid be slightly warmed. 

 He did not analyze the substance, but remarks that it presents all the 

 appearance of the brown oxide of chromium (so called). "We pre- 

 pared this compound by passing a stream of nitric oxide .through a 

 very dilute solution of bichromate of potash (a strong solution will not 

 give the reaction so readily, if, indeed, at all) for three hours ; in a few 

 minutes the liquid became dark-colored and ceased to be transparent, 

 but after standing twenty-four hours, during four of which it had 

 been gently heated, there was still no aj^parent deposit ; it was again 

 warmed, and after sixteen hours more a slimy red-brown deposit was 

 found at the bottom of the vessel. During forty-eight hours longer 

 this deposit was allowed to accumulate, when the supernatant liquid 

 was decanted and the precipitate thrown upon a filter. The decanted 

 fluid continued to deposit very slowly this red-brown matter for days 

 and weeks ; indeed, in this, and several similar experiments, we have 

 found no limit of time to the continuous, though very gradual, accumu- 

 lation of this deposit. The slimy precipitate which had been filtered 

 off was washed with hot water for twelve hours, the water at first 

 coming through of a dark-red color, but growing gradually paler till it 

 retained only a feeble yellow. The washing was continued for a week 

 with cold water, and more than nine litres of water passed through the 

 small precipitate. Whenever the color of the wash-water became so 



* Jour. Pr. Ch., XXXIX. 269 (1846). 



