OF ARTS AND SCIENCES. 343 



When dissolved, or merely suspended in dilute aqueous solutions of 

 the fixed alkalies, sesquioxide of chromium is readily converted, even 

 in the cold, into chromic acid, by the action of the peroxide of lead and 

 of manganese, by permanganate of potash, and with peculiar rapidity 

 and comjileteness by Bromine. The application of heat favors the 

 reaction in each of these instances. Iodine, also, at least when the 

 mixture is heated, appears to behave like bromine ; the oxidation is, 

 however, much less rapid. Chromic acid is also formed when red oxide 

 of mercury or hypochlorite of lime is heated with a mixture of ses- 

 quioxide of chromium and solution of caustic alkali; none was ob- 

 tained by the action of stannic or of arsenic acid, nor can minium be 

 used in place of peroxide of lead, since it does not appear to exert any 

 oxidizing action upon sesquioxide of chromium when in presence of 

 the alkalies. 



When mixed with ammonia, oxide of chromium is readily oxidized 

 by peroxide of lead, peroxide of manganese, or by solution of chame- 

 leon, the free alkali of which has been neutraUzed, if heat be applied. 

 A similar oxidation also occurs, though slowly, in the cold. With bro- 

 mine no chromic acid was obtained, — a result not at all surprising, in 

 view of the violent reaction which occurs when this substance is mixed 

 with ammonia. 



In studying the reactions which have just been described, I was met 

 at the outset by the difficulty that none of the tests for chromic acid — 

 viz. precipitation of sparingly soluble chromates of the metallic oxides — 

 which are in common use were sufficiently delicate for the purpose. 

 Indeed, besides the yellow color of solutions of the chromates, which is 

 of course far from being characteristic, there is no test for traces of 

 this acid which is susceptible of rapid and general application, except- 

 ing the formation of perchromic acid by means of peroxide of hydro- 

 gen. Having, like Reynoso, been compelled to resort to this reaction, 

 I have found it incomparably more sensitive and characteristic than 

 any of the other tests for chromic acid. Taken in connection with 

 the yellow color of solutions containing chromates, it affords a test of 

 remarkable delicacy. It depends, in brief, upon the fact, that when a 

 solution containing chromic acid is poured into a dilute * solution of per- 



* It is of the first importance that the sohitioiis used should be dilute, since no 

 perchromic acid is formed in concentrated solutions ; or if formed, it is decomposed 

 again instantly. 



