2 BULLETIN 679, U. S. DEPAETMENT OF AGRICULTUBE. 



tals that are formed."^ When it is considered how greatly crystal 

 habit may be modified by the conditions of formation, the presence 

 of impurities in the solutions, etc., the desirability of such quantita- 

 tive data is apparent.^ 



A number of crystalline alkaloids, when obtained in a pure or 

 nearly pure state, can be fairly well recognized by certain micro- 

 chemical and color reactions. When mixed, however, and particu- 

 larly when small amounts of one acccompany large amounts of another, 

 it is difficult to establish the presence and identity of each. As there 

 seemed every reason to hope that optical-crystallographic measure- 

 ments would furnish a ready and certain means for the recognition 

 of these substances, even when mixed in widely divergent propor- 

 tions, a series of observations was made by the methods outlined in 

 this bulletin. The results were satisfactory.^ 



METHODS FOR DETERMINATION OF OPTICAL CONSTANTS. 



APPARATUS. 



The instrument known as the petrographic microscope, because it 

 was developed for use in the science of petrography, the study of 

 rocks, is re'ally an apparatus for the accurate measurement of crystal- 

 lographic and optical constants of crystals in general. For instance; 

 the system to which a crystal belongs, the axial ratios of the sub- 

 stance, the refractive indices, double refraction, and optical axial 

 relations can be ascertained by its use. The determination of several 

 such features on the same crystal will naturally be of enormously 

 greater diagnostic value than the mere recording of more or less acci- 

 dental shapes assumed by a crystalline substance under ill-defined 

 conditions, which is all that is usually done. Moreover, it is not 

 necessary that large or well-developed crystals be available. Most 

 of the data can be readily measured on crystalline grains 0.025 mil- 

 limeter (^-5^ inch) in diameter, and in favorable cases on even 

 smaller ones, down to 0.01 millimeter (ytoit inch), or, in other 

 words, on what would ordinarily be classed as finely powdered 

 materials. 



While for the complete study and characterization of a crystal- 

 lized substance a somewhat elaborate microscope outfit is necessary, 

 comparatively few unusual accessories are needed to apply the 

 methods herein described to the identification of alkaloids. The 

 microscope must be equipped with nicol prisms which can be adjusted 

 so that their vibration planes are exactly horizontal (right and left) 

 and vertical (forward and backward) ; and an eyepiece with cross 



1 Kjaemer, Henry. Applied and Economic Botany (Philadelphia, 1914), p. 768. 



2 Fry, W. H. Identification of Commercial Fertilizer Materials, 1914, U. S. Dept. Agri. Bui. 97. Wright, 

 F. E. The Petrographic Microscope in Analysis, Jour. Amer. Chem. Soc, 1916, 38: 1647-1658. Chamot, 

 E. M. Chemical Microscopy, Jour. Ind. Eng. Chem., 1918, 10: 60-66. 



» To be published in the Jour. Amer. Chem. Soc. 



