230 proceedings: chemical society 



respect to certain stains aid in the diagnosis, while in other lines of 

 investigation micro-chemical reactions serve the same purpose. The 

 petrographic microscope, on the other hand, is not only a magnifier 

 but also, and essentially, a measuring instrument of precision for ascer- 

 taining the optical properties of minute crystal fragments and grains 

 a few hundredths, or even thousandths, of a millimeter in diameter. 

 After a description of the petrographic microscope, considerable stress 

 was placed on the fact that chemists had used, only to a very slight 

 degree, the quantitative aspects of crystals as a means of identifica- 

 tion. The various optical properties of crystals that offer possibilities 

 in this direction (refractive index, birefringence, optic axial angle, ex- 

 tinction angle, etc.), and their quantitative determination were briefly 

 described. The potential value of refractive index determinations for 

 identifi.cation of either pure substances as such or in crystalline mixtures 

 seems to be little realized among chemists. The method is applicable 

 to quantities (a few hundredths of a milligram) much smaller than can 

 ordinarily be subjected to chemical analysis and the time required is 

 hardly appreciable in comparison. Such measurements of the optical 

 properties of minute crystal grains have been used with great profit 

 at the Geophysical Laboratory in the identification of phases in crystal 

 mixtures and in the study of crystal growth. Of the possible lines of 

 application of petrographic microscope methods in the field of chemical 

 research some of the more important are : The identification and testing 

 of drugs, of poisons, of dyes, of chemical reagents in crystal form, of 

 chemical precipitates, baking powders, sugars and candies, spices, ce- 

 ments, abrasives; in short, of any solid material whether in powder form 

 or in larger masses, crystalline or amorphous. For homogeneity and 

 purity tests the preparation is immersed in a liquid of the refractive 

 index of the main crystal substance which then practically disappears 

 and any foreign material (adulteration or impurity) can be detected 

 at a glance. Methods are also available for estimating the relative 

 amounts of the different substances present in an aggregate. The 

 presentation was accompanied by interesting lantern projections showing 

 the appearance and behavior of minute crystals under the microscope. 



Discussion: Many members of the Society took part in the discussion, 

 arising in connection with inquiries as to the application of the methods 

 in certain fields. Such methods have been in use in the study of soils; 

 and the multiplicity of places, in the work of the chemist, where they 

 might be applicable, was soon apparent. Special emphasis was put 

 upon their use in drug identification and in the study and synthesis 

 of organic substances. It was brought out that the practical appli- 

 cability and importance of the methods will be greatly enhanced when 

 investigators have determined the optical properties of the large num- 

 ber of substances that must be considered in their use in the different 

 branches of chemistry and chemical technology. 



L. H. Adams, of the Geophysical Laboratory: Application of the 

 interferometer to the quantitative analysis of solutions. The principles 

 of working and use of the Zeiss portable interferometer for liquids were 



