OPTICAL PROPERTIES OF CRYSTALS 
255 
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. 
The chemist should be familiar with the methods used by 
crystallographers to record the optical constants and properties 
of crystals, which they describe in published papers. Even 
though simple polarizing microscopes are capable of giving but 
little of these data, there are times when advantage may be taken 
of indicated differences in optical properties to enable the analyst 
to eliminate from further consideration certain compounds which 
he at first thought might possibly be present in the material 
under examination: by way of illustration, the sodium phos¬ 
phates may be taken; we find these salts designated as follows: 
NaH2P04-H20; Orthorhombic: 2V = 29° 22', 2^ = 44° 14'; 
« = 1-4557 ft = 1-4852 7 = 1.4873 
Double refraction — negative. 
NaH2P04 • 2 H2O; Orthorhombic: 2F == 82^50', 2E = 150° 32'; 
a = 1.4401 /3 = 1.4629 7 = 1.4815 
Double refraction — negative. 
Na2HP04-7 H2O; Monoclinic: 2V = 38° 50', 2E = 57° 18'; 
a = 1.4412 /3 = 1.4424 7 = 1.4526 
Double refraction — positive. 
Na2HP04-i2 H2O; Monoclinic: 2V = 56° 43', 2E = 86° i'; 
a = I.4321 /3 = 1.4361 7 = 1.4373 
Double refraction — negative. 
Na3P04-i2 H2O; Hexagonal: w = 1.4472 e = 1.4531 
