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 6 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 following résumé 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: 
SeetramicG 2 et. fA. dc. tt 2-2 oa, : plsometric system: 
tage Tetragonal Positive. 
Uniaxial 1 eeceee iret 
BREF BG- ssn ould ts* ensantedr ) Orthorhombic) (Positive. 
Biaxial Monoclinic 
{ Triclinic Negative. 
Since light is not modified by the direction in which it traverses 
an amorphous or isometric substance, there is only one optical prop- 
