CHAPTER XL 



THE EXAMINATION OF CRYSTALLINE SUBSTANCES WITH 

 THE POLARIZING MICROSCOPE. 



The identification of most inorganic chemicals and many organic- 

 compounds is possible with a simple polarizing microscope of 

 the general type illustrated in Fig. 25 provided qualitative 

 chemical tests are also made; but in order that reliable clues as 

 to their identity may be obtained from measurements of crystal- 

 lographic constants alone, a much more elaborate instrument 

 is absolutely essential. 



This text book is intended to serve as a very elementary intro- 

 duction to the possibilities of chemical microscopy and it has 

 been thought unwise therefore to do more than point out the 

 nature of the information which may be obtained through the 

 employment of simple optical methods in the study of crystalline 

 compounds. 



To further assist the student in the application of the polarizing 

 microscope, the following brief synopsis is given to refresh his 

 memory relative to his crystallographic knowledge. 



Fundamental Crystallographic Concepts. -- According to the 

 viewpoint of the crystallographer, crystals are polyhedra, bounded 

 by plane surfaces, whose forms are dependent upon physical 

 and chemical properties and governed by the correlation of certain 

 internal forces or attractions which we may call a definite internal 

 grouping or arrangement of molecules or atoms. 



It must be remembered, however, that the chemist in recent 

 years has discovered a number of substances, appearing when 

 illuminated with ordinary light as thick syrupy liquids, yet 

 which yield optically most of the phenomena observed in solid 

 crystalline bodies. To this interesting class of compounds the 

 terms liquid crystals, crystalline liquids, or flowing crystals have 

 been given. 



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