12 Organic Acids in the Examination of Minerals. 



but few are commercial articles j some interest might be at. 

 tached to a comparison of their decomposing power, but a 

 practical application to the examination of minerals is very 

 doubtful. To test the probabilities of their action, however, 

 a few experiments were made with solutions of picric, benzoic, 

 and pyrogallic acids. 



A strong hot solution of picric acid (carefully freed from 

 nitric acid by re-crystallization) decomposes calcite, dolomite, 

 and witherite very readily. A moderately strong hot solution 

 of benzoic acid dissolves calcite freely, and a crystalline 

 precipitate falls on cooling. Dolomite dissolves on boiling. 

 Pyrogallic acid attacks calcite in the cold, and dissolves it on 

 boiling, with formation of a white precipitate, while the super- 

 natant liquid turns dark in color through absorption of oxygen. 



Microscopical Examination of the Crystalline 

 Precipitates. 



By JOHN H. CASWELL. 



10. The crystalline precipitates obtained in these reactions 

 are in some measure distinguishable by the naked eye, but 

 microscopical examination of the varied forms develops inter- 

 esting peculiarities. For this investigation we have had the 

 good fortune to secure the skill, as well as the pencil, of our 

 friend Mr. John H. Caswell, of the School of Mines, Columbia 

 College, who places us under obligation by communicating the 

 following results of his study. 



The crystalline deposits were obtained by treatment of 

 the different minerals named, with hot solutions of the acids, 

 in test-tubes, and concentration of the solution when necessary. 

 In addition to the crystalline forms of the acids themselves, 

 tartaric and oxalic, the following substances were examined : 

 Hydro-calcium tartrate, neutral calcium tartrate, hydro-barium 

 tartrate, calcium oxalate, hydro-barium oxalate, hydro-stron- 

 tium oxalate, ferrous oxalate, formate of lead, and calcium 

 citrate. The precipitates resulting from treatment of the 

 minerals were compared with similar precipitates formed from 

 chemically pure material. 



Tartaric Acid. — The most usual and characteristic forms 



