CHEMICAL MICROSCOPY 



a gross neglect of potentialities. The polariz- interference colors mainly of a higher order, 



ing microscope is necessary, and this means Even with crystals having a deep color of 



a good instrument, not just "polarizing at- their own the birefringence may be expressed 



tachments", which are no more than a make- as dim, moderate, or bright. With crystals 



shift. that are colorless or only lightly colored in 



A magnification of about 100 X is ordi- themselves the place of an interference color 



narily used, and about 200 X when a higher in the first order can be specified as precisely 



power seems advisable or necessary. as the uniformity of the crystals warrants; 



Looking at crystals through the micro- the order of higher colors can be determined 



scope, the chief characteristics of form and with the quartz wedge, 



aggregation immediately catch the eye. (See Note, on more than one crystal, whether 



"Forms of Microcrystals", p. 37.) Size, not extinction is parallel to a principal side (or 



measured but rather loosely appreciated rela- to the general direction of an irregular crys- 



tive to other crystals commonly seen, is also tal), or bisects a principal angle, or is oblique, 



obvious. Further things to observe will now The possibility of measuring an important 



be suggested. angle, at least approximately, by using the 



Note how many dimensions of the crystals revolving stage, should not be forgotten, 

 may be worth measuring. Note whether the This applies both to angles of form and to 

 crystals are fairly uniform, or show diversity the angle of extinction, 

 within one type, or whether there are two Unless the angle of extinction is close to 

 or more distinct kinds. Look over the whole 45°, if the crystals are elongate, or direc- 

 precipitate, if there are many crystals, to tional at all, the sign of elongation is impor- 

 note how some forms develop from others, tant, and finding it usually requires no more 

 Thus disks or shapeless plates when more than pushing in the red plate and observing 

 perfectly formed may be hexagons, or per- the result. With high-order interference col- 

 haps octagons. Squares and hexagons often ors it can usually be determined with the 

 skeletonize into 4- and 6-armed stars; ob- quartz wedge. All crystals can be put into 

 longs and rhomboids into X-shapes. That four groups by the sign of elongation: -1-, 

 the diamond shape is related to the hexagon — , =t, and indeterminate, 

 is frequently evident; also diamonds may Birefringence often shows whether a com- 

 extend into daggers and spears. Often very plex form is all one crystal or an aggregate 

 irregular forms can be related to a fairly of several. X-shaped crystals are usually 

 simple form. Regarding details of descrip- skeletons of a rhomboid or an oblong, along 

 tion, note particularly the ends of blades, the diagonals. The direction of the acute an- 

 rods, prisms, bars: whether square, slanting, gle shows the elongation of the parent form, 

 pointed, incised, ragged. A number of points regardless of distortions of the arms. (Fig L) 

 in regard to form have to be noted in connec- Colored crystals are nearly always due to 

 tion with birefringence. a colored reagent, and to this extent the 



Usually the next step is to look at the color does not distinguish the substance 



crystals with crossed nicols. First, note the tested, but the color is more significant the 



degree of birefringence. It will vary, of course, more it differs from the usual color produced 



with the thickness of individual crystals, but in crystals by the particular reagent. The 



very often the crystals of a precipitate will color between crossed nicols is often signifi- 



appear uniformly of much the same color, a cant. Some crystals are even known which do 



weak gray, or gray-white, or bright white to not extinguish completely but turn a differ- 



yellow, in the first order, or, on the other ent color at "extinction" positions, 



hand, nearly all may show different brilliant Dichroism is common in crystals with 



52 



