MICROSCOPIC IDENTIFICATION OF INORGANIC 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 foodstufts, 
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 
ehemical 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 ground 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.2 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. 
2 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-Microscopic 
Research, Their Relative Accuracy and Range of Application, 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., Bul. 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 Bul. 679, 1921. 
