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 go 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: 



NaH 2 P0 4 H 2 0; Orthorhombic: 2 V = 29 22', 2E = 44 ° 14'; 



« = 1-4557 = 1.4852 7 = 14873 

 Double refraction — negative. 



NaH 2 P0 4 -2H 2 0; Orthorhombic: 2V = 82 50', 2E = i50°32 r ; 



a = 1. 4401 j8 = I.4629 7 = 1. 4815 



Double refraction — negative. 



Na 2 HP0 4 -7 H 2 0; Monoclinic: 27 = 38° 50', 2^ = 57° 18'; 



a = 1. 4412 j3 = I.4424 7 = 1.4526 



Double refraction — positive. 



Na 2 HP0 4 -i2 H 2 0; Monoclinic: 2V = 56 43', 2E = 86° 1'; 



a = 1.4321 /3 = 1.4361 7 - 1-4373 

 Double refraction — negative. 



Na3P0 4 i2 H 2 0; Hexagonal: w = 1.4472 e = 1.4531 



