OBSERVING MICROCRYSTALS 



some colored reagents, giving not merely the 

 fact of dichroism to distinguish certain sub- 

 stances, but two different colors to be ob- 

 served and specified. Some crystals (mostly 

 with iodine reagents) are pleochroic, with 

 three extreme colors, and as seen on the slide 

 usually shoAV quite a variety of colors with 

 polarized light, changing with rotation of the 

 stage. Pleochroism is the general term (in- 

 cluding dichroism), but as there are usually 

 only two quite different colors, and moreover 

 an individual crystal as it lies on the slide 

 can only show two extreme colors with rota- 

 tion of the stage, the writer prefers to use 

 the term dichroism when it is applicable. 



Dichroism can usually be noted merely by 

 rotating the stage, but when it is feeble it 

 may be necessary to test the extinction posi- 

 tions, which show the extreme colors, to ob- 

 serve it. A crystal is turned to extinction 

 position between crossed nicols, then ob- 

 served using only the polarizer or only the 

 analyzer, then observed again in the next 

 extinction position, at right angles. The 

 change in color may be slight, or great; di- 

 chroic microcrystals are known with iodine 

 reagents which change from slight yellowish 

 to black, with bromauric acid which change 

 from pale yellowish (sometimes appearing 

 colorless) to deep bright red, with iodopla- 

 tinic acid which change from pink to dark 

 blue, or from green to purplish red. 



Note not only the two different colors, 

 but also their orientation. The crystal is said 

 to have a positive sign of absorption when 

 the darker color is "lengthwise", negative 

 when it is "crosswise". The sign of absorp- 

 tion, which also depends on elongation, 

 nearly always agrees with the usual sign of 

 elongation, when both can be observed. 



Dichroic crystals often show a peculiar 

 quality of Color in ordinary light, and also 

 show the deep dichroic color where they 

 overlap at right angles. Pleochroic (tri- 

 chroic) crystals may be recognizable even in 

 ordinary light. 



Observe the relation of various forms to 



Fig. 1. dZ-Amphetamine with HAuCli in (1 + 

 2) H3PO4 , applied directly to tablet material. 

 Crossed nicols. 66X . The X-crystals show negative 

 elongation. 



birefringence. Hexagons may or may not 

 show birefringence. When they do not show 

 it, lying flat, there are often interspersed rods 

 that do. Crystals appearing square or four- 

 parted may be isotropic, or only some of 

 them may show birefringence, in this case 

 usually not very strong even when present 

 (interference figures possible). All may show 

 definite birefringence — even quite high — 

 and may extinguish parallel to the crosshairs 

 in some cases, or diagonally with crystals of 

 a different kind. Thus crystals of the same 

 form may be of quite different types when 

 birefringence is also considered. 



Examine for interference figures — this 

 requires at least a 20 or 21 X objective — 

 when sizable, transparent cr.ystals show only 

 low or no birefringence, especially if in the 

 latter case the crystals show birefringence 

 when tilted, or some of them are more or 

 less birefringent (tilt of the axes in the 

 crystal), or are accompanied by other crys- 

 tals (possibly of the same crystal system, 

 but a different elongation) which are quite 



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