264 ELEMENTARY CHEMICAL MICROSCOPY 



Practical application may be made of the phenomenon of 

 pleochroism in differentiating between different textile fibers and 

 different paper fibers stained with certain aniline dyes. Some 

 species of fiber exhibit strong pleochroism and others weak. 



The Measurement of Crystal Angles and Extinction Angles. — 

 Since the interfacial angles of crystals of chemical compounds are 

 always constant for similar faces no matter how the compound 

 may have been prepared, it is obvious that angle measurements 

 may often prove of the greatest value in the identification or dif- 

 ferentiation of compounds or of crystal systems. When crystals 

 are of sufficient size to be handled determinations of the values 

 of angles by means of some form of goniometer are fraught with 

 no great difficulties, but when the crystals are microscopic and 

 cannot satisfactorily be orientated, the problem becomes exceed- 

 ingly difficult. 



Fortunately, the chemist is rarely if ever called upon to make 

 very accurate angle measurements; rapid approximate readings 

 are usually sufficient for analytical work. Moreover, so-called 

 chemical microscopes are incapable of yielding angular measure- 

 ments of the degree of accuracy required in crystallographic 

 investigations. 



Great accuracy on the part of the analyst is seldom essential, 

 since his object is merely to ascertain whether the crystal under 

 examination is, or is not, a certain compound. In simple inor- 

 ganic analyses angle measurements are rarely resorted to, but 

 in the examination of organic compounds and in the case of mix- 

 tures of inorganic and organic substances, the measurement of 

 angles may often prove a most rapid means of differentiation. 



Only thin, well-formed crystal plates with practically perfect 

 edges should be selected for measurement. Avoid high magni- 

 fications. The rotating stage having been previously centered, 

 the preparation is moved with the fingers until the selected 

 crystal is brought under the cross-hairs of the eyepiece. One of 

 the bounding edges of the angle sought is placed exactly parallel 

 to and almost in coincidence with one of the cross-hairs ; the posi- 

 tion of the graduated circle of the stage is noted and the stage is 

 rotated until the other bounding edge of the angle becomes par- 



