214 PROCEEDINGS OF THE AMERICAN ACADEMY 



had been sufficient to make it somewhat basic. The point specially 

 noticeable in Rammelsberg's statement is his assertion that the precipi- 

 tate was. washed till it imparted no color to cold water. This is not 

 credible, for the testimony of all other chemists is unanimous upon the 

 point, that there is no limit to the removal of chromic acid by washing 

 from this precij^itate. We have often prepared much more basic pre- 

 cipitates, and have never yet seen colorless wash-water from the most 

 basic of them. The color may become so pale as not to be very no- 

 ticeable in small vessels. 



The discussion which we have traced left the subject in such doubt 

 and obscurity, that most writers of text-books have given both views of 

 the substance in question, some leaning to one theory of its composition, 

 some to the other. In our endeavor to settle the question, we shall next 

 bring to bear upon the subject the arguments to be di'awn from analogy. 



II. Analogous Chromates. 



The metals with which chromium is allied are aluminum, iron, and 

 manganese, and the existence of chromates of the sesquioxides of 

 these metals with properties analogous to those which we have de- 

 scribed as belonging to the chromate of chromic oxide, will be addi- 

 tional evidence that this substance is rightly so called. 



1. Chromate of Ahimina. Maus* observed, but did not analyze, 

 the chromate of alumina which is precipitated when chromate of potash 

 is added to a solution of alumina in chromic acid ; he speaks of it as 

 " consisting of chromic acid with much alumina." When the precip- 

 itate was thoroughly washed, pure hydrate of alumina remained on the 

 filter and acid chromate of alumina passed into the filtrate. Fairrie f 

 also describes a chromate of alumina prepared by mixing alum solution 

 with the solution of chromate of potash, and his analysis led to the ex- 

 act formula AI2O3 CrOg ; he states, however, that the precipitate was 

 thoroughly washed, which in connection with the result of his analysis 

 is quite incomprehensible ; for Maus observed, and our own experiments 

 fully confirm his observation, that all the chi'omic acid may be readily 

 washed out of this compound. 



The yellow precipitate which appears when chromate of potash is 

 dropped into a solution of alum, is constantly re-dissolved until the pro- 

 portion of chromate of potash added amounts to five equivalents to each 



* Fogg. Ann., XI. 82 (1827). t Jour. Chem. Soc, IV. 301 (1852). 



