MICRO-ANALYSIS. 819 



(Fig. 292). Again upon dissolving the product either in water, 

 as with catechu (Fig. 275, A) or in solutions of chloral, as with 

 gambir (Fig. 275, B) a crystalline residue remains. Finally 

 upon extracting the dried drug with suitable solvents as ProUius' 

 solution and evap'orating the solvent, characteristic crystals 

 separate as with coca, hydrastis, nux-vomica, cinchona, cola, 

 guarana, etc. ; or distinct crystalline precipitates may be obtained 

 upon addition of special reagents as palladous chloride to solu- 

 tions containing cocaine hydrochloride (Fig. 157), or gold 

 chloride to solutions containing cafifeine (Fig. 149). 



Cognizance of these crystals is being taken to some extent 

 in all of the progressive pharmacopoeias and in numerous papers 

 published during the past ten years. The subject is in a more 

 or less chaotic condition at present and pharmacognosists must 

 recognize the importance of the careful study of these micro- 

 scopic crystals. Superficial descriptions and color reactions will 

 not alone serve to identify these substances. Like the miner- 

 alogist in the study of rock sections the pharmacognosist must 

 employ the petrographical microscope, and it is at once apparent 

 that sooner or later the principles of physical and chemical 

 crystallography must be employed by students in pharmacy as 

 well as by the analyst. The work is by no means so simple as 

 in ordinary microscopic work, but when the principles governing 

 the optical study of crystals are mastered, the study will be 

 found quite as satisfactory, and will appeal to the pharmacist 

 not only as a fertile field for research, but as one of the most 

 promising methods for the identification of drugs and affording 

 an important clue to their quality and real vgiluation. 



The study of microscopic crystals is accomplished by means 

 of the petrographical microscope. Brown (loc. cit.) has stated 

 succinctly the nature and use of this instrument : 



" The necessity of studying small crystals, . . . has re- 

 sulted in the evolution of a form of microscope which is at once a 

 goniometer, a polariscope, and an instrument for measuring 

 optic axial angles in short, for determining the physical crys- 

 tallographic constants of small crystals. . . . The polari- 

 scope portion of the petrographical microscope enables the ob- 

 server to determine the position and relative value of the elasticity 



