OF ARTS AND SCIENCES. 193 



cess deepened the yellow color of the wash-water in each case. It being 

 quite evident that the color of the wash-water was due to chromic acid 

 abstracted from the precipitate, the washing was stopped on the eighth 

 day, the precipitate was dried at 50° to 55°, and analyzed as follows. 

 A portion was dissolved in very dilute nitric acid by the aid of a gentle 

 heat ; the color of the solution was a red brown. Ammonia, added in 

 very slight excess to this solution, kept at the boiling point for half 

 an hour, produced a brownish-green precipitate of chromic oxide, which 

 looked the bi'owner because it floated in a bright yellow liquid. This 

 precipitate was filtered off and thoroughly washed with hot water ; it 

 then presented the common appearance of chromic oxide, and was 

 ignited and weighed in the usual manner. The filtrate from this pre- 

 cipitate and the wash-waters were concentrated by evaporation, and 

 acidified with acetic acid. Acetate of lead then precipitated the whole 

 of the chromic acid contained in the yellow liquid, and after the sub- 

 sidence of the precipitate the supernatant liquid was perfectly colorless. 

 The chromate of lead was washed and weighed on a tared filter as 

 usual. The result of the analysis was 



CrOs = 16.27 per cent 



CrA = 46.43 " " 



Water = 37.30 " " (by difference). 



This examination showed, first, that the washed precipitate was a 

 compound of chromic acid and chromic oxide ; secondly, that it was 

 more basic than any supposable definite compound of these two bodies ; 

 thirdly, that the prolonged washing removed chromic acid from the sub- 

 stance originally precipitated. To study the effect of the washing 

 upon the precipitate, we prepared a considerable quantity of the brown 

 precipitate by mixing a concentrated solution of pure chrome alum 

 with a large excess of a concentrated solution of monochromate of 

 potash. The precipitate as fii'st collected upon the filter of course re- 

 tained a considerable quantity of the bichromate of potash, chromate of 

 potash, and sulphate of potash which existed in the filtrate. The pre- 

 cipitate was quickly rinsed with cold water until these mechanically 

 adhering salts seemed to have been removed. During this short wash- 

 ing the substance underwent no change in color or in any other exter- 

 nal property. A portion was now removed from the filter, air-dried 

 on a brick, and submitted to analysis (analysis a). The rest of the 

 precipitate was washed for six hours with cold water ; another portion 



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