10 Organic Acids in the Examination of Minerals. 



need only add that the intensity of action claimed for it is fully 

 maintained by later researches. 



(d) All the minerals of the first group, 25 in number, with 

 three exceptions, dissolve in the nitro-citric mixture 

 rapidly and completely, several of them yielding solu- 

 tions of characteristic color. Even sulphur itself is 

 decidedly attacked, with formation of sulphuric acid. 

 The exceptions are realgar, orpiment, and proustite. 



(e) Two of these, realgar and orpiment, are partially de- 



composed by boiling with the iodo-citric mixture. 

 Proustite and sulphur resist even on prolonged heating. 

 All the remaining minerals of this group are quite 

 readily dissolved, the decomposition of the sulphides 

 being accomjDanied by liberation of sulphuretted hy- 

 drogen. 

 The differentiating power of these solvents is again exhibited 

 by these reactions. In our first paper we showed, that while 

 bornite and pyrrhotite are decomposed by citric acid, their kin- 

 dred compounds, chalcopyrite and pyrite, are not (13). We 

 now find that proustite resists completely the decomposing solu- 

 tions named, while pyrargyrite is decidedly attacked by the 

 nitro-citric mixture as well as by the iodo-citric mixture, even 

 in the cold. 



This difference in behavior of the two closely allied minerals 

 was established by numerous determinations. 



Oxides. 



30. The oxides examined include such stable minerals as co- 

 rundum, spinel, chrysoberyl, cassiterite, rutile, hyalite, and 

 quartz, which naturally resist these methods of attack. 



Gummite is attacked by cold citric acid, and melaconite and 

 goethite are soluble to a certain extent on heating. Menaccanite 

 and washingtonite are feebly attacked by the nitro-citric mixture 

 and strongly by the iodo-citric solution. This latter also 

 strongly attacks braunite and goethite. 



