266 ELEMENTARY CHEMICAL MICROSCOPY 



graphic direction usually adopted by chemists, where the extinc- 

 tion angle is employed as one of a series of identity tests, is the 

 longest edge of the crystal or in the case of rhomb-shaped crys- 

 tals the line bisecting the acute angles. 



In the case of crystals exhibiting parallel extinction the extinc- 

 tion angle may be considered as being o degrees. Crystals 

 exhibiting oblique extinction, i.e., those of the monoclinic and 

 triclinic systems yield two extinction angles; but it is customary 

 to record as the extinction angle the smallest angle obtained 

 between the length of the crystal (cleavage lines or edges being 

 used), and the nearest axis of elasticity. In 

 Fig. 144 the extinction angles may be consid- 

 ered as the angles 6. 



If the analyst is sufficiently well trained in 

 crystallography to be able to locate the c-axis 

 he may record as the extinction angle the 

 angle formed between the c-axis and the 

 nearest axis of elasticity. This value is that 



most often taken by crystallographers as the 

 FIG. 144. Extinction . . . J . J 



Angles, 0, e. characteristic extinction angle. 



Since in feebly polarizing crystals the exact 

 point of extinction is not easily determined, a measurement of 

 the angle is difficult and annoying unless a selenite test plate is 

 employed (see page 261). When employing a selenite proceed 

 as follows: Place the test plate, red of the first order, so that 

 the plane of its direction of vibration bisects the opposite quad- 

 rants of the cross-hairs of the ocular. With the nicols crossed 

 bring a typical thin crystal so that its long edge (or its c-axis) 

 lies parallel to a cross-hair. A red field is seen with the crystal 

 of some contrasting color. Read the graduated stage circle. 

 Now slowly rotate the stage until the crystal acquires exactly the 

 same color as the field; the plane of vibration of the selenite and 

 that of the crystal are now coincident. Read the stage again. 

 The reading will give an extinction angle. Next ascertain whether 

 it is the smaller of the two possible angles for this position of 

 the crystal. Make similar measurements upon a number of other 

 crystals. 



