Organic Acids in the Examination of Minerals. 19 



guish pyrrhotite from pyrite, and bornite from chalcocite or 

 chalcopyrite. The presence of galenite in argentite may also 

 be ascertained by this method, since the former yields H 2 S 

 even in the cold, while the latter is not decomposed on 

 boiling. 



OXIDES. 



14. A few of the mineral oxides examined are attacked by 

 the organic acids in the cold ; brucite dissolves slowly, but to 

 a considerable extent, in cold citric acid; the manganese 

 oxides, hausmannite, pyrolusite, manganite, psilomelane, and 

 wad, decompose the organic acid with evolution of carbonic 

 anhydride ; the reaction begins in the cold, and on applying 

 heat proceeds rapidly, with a lively effervescence. 



9 MnO 2 +10 H 3 C 6 H 5 7 =9 MnHC 6 H 5 7 +13 H 2 0+6CO s 



JManganite is not so soluble as the others, probably because 

 the oxidizing power of the sesquioxide is less than that of 

 manganese dioxide. The minerals are apparently more 

 quickly and completely dissolved by oxalic acid than by citric 

 acid. 



Zincite, cuprite, and limonite are attacked by boiling citric 

 acid, the latter but slightly. Hematite, magnetite, franklin ite, 

 and chromite are not attacked. In testing these minerals, 

 conclusions were based on the reactions of the filtrates; in 

 employing ferrocyanide of potassium to test the citric acid 

 solutions of the iron oxides, we observed that the organic acid 

 exerts a reducing action on the ferrocyanide, and produces a 

 bluish precipitate proceeding from this reagent itself. This is 

 especially noticeable on boiling, a light bluish-green precipi- 

 tate forming abundantly. Tartaric and oxalic acids act 

 similarly. 



The oxides behave with tartaric acid in all respects as with 

 citric. 



SUNDRY MINERALS. 



15. A few minerals not closely related were examined as 

 to their behavior with citric acid. Gypsum appears to be 

 somewhat more soluble in a concentrated solution of citric 



