Organic Acids in the Examination of Minerals. 27 



cold with liberation of sulphureted hydrogen ; D, those which 

 dissolve in the hot acid without evolution of gas; E, those 

 which dissolve in the hot acid with liberation of carbonic 

 anhydride; F, those which dissolve in the boiling acid with 

 liberation of sulphureted hydrogen; G, those which are de- 

 composed on boiling, with formation of gelatinous silica ; H, 

 those which are decomposed on boiling, with separation of 

 silica in a slimy or a pulverulent form. To these eight groups 

 we may add three more ; I, those which are decomposed by 

 boiling with citric acid and potassium nitrate ; K, those which 

 are decomposed by heating with citric acid and ammonium 

 fluoride ; and L, those which are not attacked by any of the 

 preceding methods. 



Under E fall five minerals, hausmannite, manganite, psi- 

 lomelane, pyrolusite, and wad, which dissolve rapidly in hot 

 citric acid and decompose it with liberation of carbonic anhy- 

 dride. Advantage is taken of this reaction in the quantitative 

 analysis of manganese dioxides. 



Notwithstanding the sharpness of the reactions by which 

 many of the minerals are characterized, we do not regard the 

 annexed table as a scheme for their determination, but merely 

 as a general view of their behavior. The special cases in 

 which minerals may be distinguished have been mentioned 

 previously. 



Under L are found thirteen of the ninety minerals exam- 

 ined; perhaps some of these will yield to these methods of 

 attack by varying slightly the means of application. It is 

 possible also that some of them may be decomposed by heat- 

 ing with citric acid solution in sealed tubes under pressure ; 

 but this process of course is not applicable to field work. 



