PLEOCHROISM — CRYSTAL ANGLES 263 



been subjected to chemical treatment. As an example of the 

 latter, there may be cited, the change in sign from + to — in 

 nitrocellulose as the percent of nitration increases. In nitro- 

 cellulose low in nitrogen the double refraction is positive, but 

 nitrocelluloses high in nitrogen show negative double refraction; 

 the change is a gradual one, the transition point being between 

 nitrogen contents of n and 12 per cent. 1 It is obvious that 

 the polarizing microscope affords a convenient method of ascer- 

 taining the degree of nitration of a given sample of nitrocellulose. 



Absorption. Pleochroism. Many compounds have the 



power of absorbing part of the light rays vibrating in certain 

 planes and therefore if viewed through the polarizing microscope 

 with the analyzer removed will exhibit a change of light intensity, 

 in certain positions. This property of crystals known as absorp- 

 tion should not be confused with a change of color. 



All anisotropic substances to a greater or lesser extent remove 

 the rays of certain colors in certain planes from white light sent 

 through them. This property when sufficiently pronounced to 

 be observable with the normal human eye is termed pleochroism. 

 Substances are tested for pleochroism by placing them upon 

 the stage of a polarizing microscope, removing the analyzing nicol 

 and rotating the polarizer. If the substance under examination 

 is pleochroic, it. will change in color with the rotation of the prism. 

 In the event of the polarizer being fixed and incapable of rotation, 

 rotate the stage. Always carefully shade the preparation with 

 the hand in order to prevent as much as possible confusing 

 reflections. 



If the phenomena observed involve a two-color change the 

 crystals are said to be dichroic; if a three-color change trichroic. 

 Uniaxial crystals can exhibit only a two-color change; biaxial 

 crystals may be trichroic. 



Isotropic crystals possessing a high adsorption power for cer- 

 tain coloring matters may become in the process of their growth 

 highly colored. These crystals, although still retaining their 

 isometric habit are often highly pleochroic. 



1 Chardonnet: Zeit. ang. Ch. 1899, 31. Lunge and Bebic; Zcit. ang. Ch. 1901, 

 567. Ambrom: Koll. Zeit. 13 (1913), 200. 



