Organic Acids in the Examination of Minerals. 7 



3. The organic acids employed in this investigation were 

 chiefly citric, tartaric, and oxalic ; a few tests were made also 

 with malic, formic, acetic, benzoic, pyrogallic, and picric acids. 

 Of the solid acids, solutions saturated in the cold were used, 

 unless otherwise specified ; of the liquid acids, ordinary com- 

 mercial products. The behavior of the minerals with these 

 acids was studied in a very simple manner : the mineral to be 

 examined was carefully freed from its associated gangue or 

 mineral, finely pulverized in an agate mortar, and a portion 

 placed in a test-tube ; the solution of the acid was then added, 

 and the resulting phenomena, iu the cold and on boiling, care- 

 fully noted. Sometimes satisfactory conclusions were reached 

 only by comparison of a number of tests under varying condi- 

 tions, as to amount of acid, time of heating, etc. In some 

 cases, to be mentioned in due course, the partial decomposition 

 of the mineral was ascertained by filtering from the residue 

 and testing the solution with an appropriate reagent ; in 

 others, by examining the disengaged gas with a suitable test- 

 piiper. 



CARBONATES. 



4. The natural carbonates dissolve with effervescence more 

 or less readily in dilute and strong, cold and hot solutions of 

 citric, tartaric, oxalic, malic, formic, benzoic, acetic, pyrogallic, 

 and picric acids, the relative power of these acids being 

 approximately in the order in which they are named. The 

 behavior of the carbonates with citric acid may be summarized 

 as follows : 



(a) Calcite, gurhofite, witherite, strontianite, cerussite, baryto-calcite, 

 and malachite dissolve rapidly in the cold. 



(5) Dolomite, ankerite, rhodochrosite, smithsonite, and azurite are more 

 feebly attacked in the cold. 



(c) Magnesite and siderite are not attacked in the cold. 



On heating, all the above carbonates dissolve very rapidly, 

 except siderite, which is more slowly attacked. From strong, 

 nearly neutral solutions of calcite, gurhofite, and some other 

 calcium minerals, a white precipitate of calcium citrate forms 

 on cooling; cerussite also deposits a white precipitate on 

 cooling. 



