172 ELEMENTARY CHEMICAL MICROSCOPY 



are uniaxial. Those of the orthorhombic, monoclinic and 

 triclinic systems are biaxial. When doubly refracting crystals 

 lie in such a position that their optic axes are parallel to the 

 optic axis of the polarizing microscope, the nicols being crossed, 

 the crystals remain dark when the stage is rotated; in other 

 positions the crystals will appear alternately bright and dark. 

 The optic axis of a crystal may thus be found experimentally. If 

 an anisotropic crystal remains dark (on turning the stage) in one 

 position only it is uniaxial; if dark in two positions it is biaxial. 



To obtain a clue as to the probable system of a substance 

 yielding polarizing crystals, find the position of extinction, read 

 the stage and remove the analyzer. Now turn the stage until 

 the centered crystal has its crystal boundaries or crystal cleav- 

 age lines lying coincident with the cross-hairs. Read the stage 

 again. Try a number of crystals in turn. If the angle is o 

 degrees or 90 degrees, in all the crystals, the system is either 

 tetragonal, hexagonal or orthorhombic, i.e., the crystals exhibit 

 parallel extinction. If the angle is not-o degrees or 90 degrees 

 the crystals are monoclinic or triclinic. 



The tetragonal or hexagonal systems are not to be differen- 

 tiated save through their crystal form and crystal cleavage. 



Directions of Vibration, or Axes or Directions of Elasticity. 

 In all the doubly refracting crystals there are certain directions 

 through them in which the light rays advance or are transmitted 

 with a greater velocity than in other directions. 



"The directions of vibration (found always to be at right 

 angles to each other) of the light rays which advance with maxi- 

 mum or minimum velocity and a third direction at right angles 

 to the plane of these directions (corresponding to some ray with 

 an intermediate velocity) are called Axes of Elasticity " l 



In the orthorhombic system the axes of elasticity coincide with 

 the crystallographic axes. 



In the monoclinic, one axis of elasticity coincides with the 

 b-axis, the other two axes of elasticity are in a plane of symmetry 

 at right angles to b, but are coincident with neither the c-axis nor 

 the a-axis. 



1 Luquer, Minerals in Rock Sections. New York, 1808. 



