I 



348 PROCEEDINGS OF THE AMERICAN ACADEMY 



It must be remembered, however, that this coloration, although a 

 very delicate, is by no means a characteristic test,* and that the pres- 

 ence of even considerable quantities of chromic acid cannot be de- 

 tected in the solution by the methods in common use. For example 

 the yellow-colored filtrate obtained after having boiled a portion of the 

 precipitate from mixture No. 3 (which contained more than 2.5 per 

 cent of sesquioxide of chromium) with a solution of caustic soda and 

 some peroxide of lead, was slightly acidified with acetic acid, care 

 being taken not to dilute the solution unnecessarily. No precipitate 

 was formed even after the lapse of forty-eight hours ; the solution still 

 retained its yellow color, however, and afforded a fine blue ethereal 

 solution when treated with peroxide of hydrogen. Even in experi- 

 ment No. 1, the yellow alkaline solution of chromate of lead gave no 

 immediate precipitate when neutralized with acetic acid, nor did any 

 appear until after the lapse of considerable time. 



When concentrated alkaline solutions are used in conjunction with 

 the peroxide of lead, it is best to acidify them, before testing with 

 peroxide of hydrogen, since the reaction might otherwise be disturbed. 

 This is not necessary, however, when dilute alkali is used. 



The methods of oxidizing chromium which have been discussed in 

 this article are of course applicable to a great variety of cases. I 

 have only specially treated of the one in which this oxide is concealed 

 by an excess of peroxide of iron, since an improvement upon the ordi- 

 nary processes seems to be peculiarly needed in this instance. It will 

 be found, however, that, with the exception of the blowpipe tests, none 

 of the methods for the detection of chromium which are now in use 

 can be compared with the one herein proposed. 



An apparent objection to this process is the doubt which suggests 

 itself whether peroxide of hydrogen may not be capable, under some 

 circumstances, of producing perchroraic acid when mixed with solu- 

 tions, not only of chromic acid, but also of simple sesquioxide of 

 chromium. It is indeed somewhat strange that this should not be 

 the case. I have not, however, been able to effect such transforma- 

 tion, although I have made a large number of experiments with so- 

 lutions and mixtures of the sesquioxide in the mineral acids and in 

 alkalies, under the most varied conditions as regards their temperature 



* It is probable that by the use of the prism, as proposed by Gladstone (Qu. J. Ch. 

 See., X. 79), these colored solutions might be satisfactorily tested for chromic acid. 



