MICROSCOPIC IDENTIFICATION OF INORGANIC SALTS. 5 



tal faces and axes. Pleochroism should be observed when present, 

 and crystal system should always be identified where possible. 

 The arrangement is according to ascending values of the /3 index. 



OPTICAL PROPERTIES. 



It can not be too strongly stated that geometrical and optical crys- 

 tallography constitute two distinct classes of crystal investigations; 

 closely interrelated, it is true, and mutually dependent for certain 

 properties, but nevertheless far from being completely expressible in 

 terms of each other. For example, a complete geometrical descrip- 

 tion of a crystal could do no more than indicate isotropy, uniaxiality 

 or biaxiality, leaving all of the other optical properties completely 

 undecided. 



To show the relations between geometrical and optical crystallog- 

 raphy and to explain briefly the optical terms and methods here 

 used, the folloAving resume is given. 



All crystals can be grouped into six major systems, each of which 

 has numerous subdivisions or modifications. These major systems 

 are the isometric, tetragonal, hexagonal, orthorhombic, monoclinic, 

 and triclinic. Optically, these may be grouped into two classes, 

 depending upon the mode of transmission of light in them in different 

 directions. Substances belonging to the isometric system transmit 

 light with equal speed, regardless of crystallographic directions. 

 They therefore act very much as a piece of glass or other amorphous 

 body. Such substances form the isotropic class. All belonging to 

 the other systems transmit light with a speed which varies as the 

 direction through the crystal varies. These form the anisotropic, 

 doubly refracting, or birefringent class. 



Anisotropic materials comprise two main groups, the first having 

 only one optic axis, and the second having two. These are the uni- 

 axial and biaxial groups, respectively. The uniaxial groups include 

 the tetragonal and hexagonal systems, and the biaxial group the 

 orthorhombic, monoclinic, and triclinic systems. The uniaxial and 

 biaxial groups are further subdivided into optically positive and 

 optically negative subgroups. 



We therefore have an optical classification as follows; 



Isotropic Isometric system. 



'iTniaxial /Tetragonal \ /Positive, 

 uniaxial ^Hexagonal / \Negative. 



Anisotropic ^^ fOrthorhombic] [Positive. 



Biaxial | Monoclinic > I 



[ [Triclinic J [Negative. 



Since light is not modified by the direction in which it traverses 

 an amorphous or isometric substance, there is only one optical prop- 



