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 PO4 
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 H2O 
or that with 12 H2O. 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.” ^ 
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. 
’ Luquer, Minerals in Rock Sections. New York, 1898. 
