CHAPTER XI. 
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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