OF ARTS AND SCIENCES. 207 



to a little higher temperature than the preceding one, in order to exhibit 

 the effect of different temperatures in the production of chromic acid. 

 A portion (No. 1) of the evaporated residue was heated on a porcelain 

 dish at a low temperature, with constant stirring, to complete dryness ; 

 a second portion (No. 2) was first thoroughly dried at the same tempex'- 

 atui'e to which No. 1 had been exposed, and was then somewhat more 

 strongly, but still very gently, heated for a few moments, during which 

 nitrous fumes escaped in abundance ; in this manner each substance 

 was dried at the highest temperature to which the preceding one had 

 been exposed, and was then subjected to a little stronger heat during 

 five or fen minutes ; the last substance (No. 5) was heated as hot as 

 was possible, while avoiding incipient redness. The chromic acid of 

 each substance was determined by precipitating it with nitrate of the 

 suboxide of mercury ; the chromic oxide was also determined in two or 

 three cases, but only to control the analyses. All five substances were 

 soaked in water for five hours ; pure nitric acid was then added in very 

 small quantity, and, at the end of forty-four hours more, solution had 

 been effected in each case. The color of the dry powder was brownish 

 black, that of the solutions brownish yellow. The percentage of chromic 

 acid in each substance was as follows : — 



It therefore appears that a variable amount of chromic oxide may be 

 converted intp chromic acid by evaporation with nitric acid, and that 

 the quantity so changed increases with the increase of temperature till 

 incipient redness is reached. If this mixture of chromic oxide, chromic 

 acid, and nitric acid be exposed to a red heat, the nitric acid and the 

 absorbed oxygen are driven off together, and nothing remains but insol- 

 uble chromic oxide. It is sufficiently clear from these results, that no 

 definite compound can be formed during this evaporation of nitric acid 

 in contact with chromic oxide ; the result is simply an indeterminate 

 mixture of chromic oxide and chromic acid. It may be remarked, in 

 passing, that this mixture closely resembles in every external property 

 the brownish black powder obtained by gently heating chromic oxide in 



