MICROSCOPIC IDENTIFICATION OF INOEGANIC SALTS. 3 



to identify the various substances occurring in commercial fertilizers 

 and the salts obtained from the evaporation of soil extracts. Other 

 minor applications have been the examination of scouring soaps, 

 products of dye manufacture, ground glass and sand in foodstuffs, 

 incrustations and efflorescences on buildings, gas pipes, soils, etc., 

 chemical precipitates, miscellaneous laboratory reagents, and com- 

 mercial products such as ordinary salts and carborundum. Possibly 

 one of the widest applications may be in the inspection of drugs and 

 similar products. 



The petrographic microscope was primarily designed for the identi- 

 fication of natural minerals. But since minerals are nothing more 

 than a particular group of more or less definite and usually impure 

 chemical compounds which occur in nature, the petrographic micro- 

 scope is all the more applicable to the crystalline compounds of the 

 laboratory where purity is usually of a fairly high degree. The 

 methods have an immense advantage over ordinary microscopic 

 observations of crystals in that particles showing crystalline forms or 

 outlines are not essential. The petrographic determinations are made 

 upon the optical constants with only incidental and occasional 

 reference to crystallographic constants, and therefore can be made 

 upon crushed or groimd fragments showing no crystalline outlines 

 almost as well as upon well-developed crystals. Furthermore, the 

 optical methods are accurate and certain to a degree which can not 

 be attained in a measurement of crystals under the microscope. 



The optical properties have been repeatedly described and the 

 methods for their determination given. ^ They are isotropy, or 

 anisotropy, uniaxiality or biaxiality, optical character, refractive 

 indices, birefringence, optic axial angles, dispersion, color, pleo- 

 chroism, absorption, orientation, and such crystallographic properties 

 as cleavage and system where possible. Isotropy, anisotropy, uni- 

 axiality, biaxiality, and optical character are qualitative in their 

 nature and serve simply to throw a particular substance into a classi- 

 ficatory group. All the other properties are specific for a given sub- 

 stance and serve to identify it when once placed in its proper group. 

 Dispersion, color, pleochroism, and absorption are essentially qualita- 

 tive as usually observed. Refractive indices, birefringence, optic 

 axial angles, and orientation are distinctly quantitative, however, and 

 refer to specific numerical values. 



* For an admirable and exhaustive account of methods see Johannsen, Manual of Petrographic Methods, 

 New York, 1914. For a general critique of methods see Wright, The Methods of Petrographic-Mieroscopic 

 Research, Their Relative Accuracy and Range of AppUcation, Carnegie Institution of Washington, Pub. 

 158, 1911. An extremely elementary account of the simplest and most easily applied methods has been 

 given by Fry, Identification of Commercial Fertilizer Materials, U. S. Dept. of Agr., Bui. 97, 1914. Among 

 numerous other publications may be mentioned the classic works of Iddings, Rock Minerals, New York, 

 1906; Rosenbusch and Wulfing, Mikroskopische Physiographie der petrographisch wichtigen Mineralien, 

 1 Teil, Stuttgart, 1904; Schroeder van der Kolk, Kurze Anleitung zur Mikroskopischen Krystallbestim- 

 mung, Wiesbaden, 1898; and Larsen, U. S. Geol. Survey Bui. 679, 1921. 



