256 ELEMENTARY CHEMICAL MICROSCOPY 



Let us suppose that the chemist finds on making a qualita- 

 tive analysis that a certain crystalline salt contains Na and PC>4 

 ions only and that he wishes to ascertain which sodium phos- 

 phate he has in hand. Three of them give parallel extinction 

 in all positions, two of them oblique in one position. Obviously 

 a determination of the character of the extinction exhibited by 

 the salt in question will not at once show whether it is a disodium 

 phosphate or not. Suppose the data obtained indicated that 

 the salt is either one of the monosodium phosphates or is the 

 trisodium salt, then refractive index determinations will prob- 

 ably show whether it is mono or tri. If further an interference 

 figure can be obtained the problem is at once solved. If on the 

 other hand the observations indicate that the crystals are prob- 

 ably monoclinic, again a refractive index determination will 

 show the analyst whether he has in hand the salt with 7 H^O 

 or that with 12 H^O. Or in this case a determination of the 

 optical character of the crystal whether positive or negative 

 will also solve his problem. 



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 

 maximum 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." 1 



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. 



In the triclinic system no axis of elasticity is parallel with a 

 crystallographic axis. 



1 Luquer, Minerals in Rock Sections. New York, 1898. 



