CHEMICAL iMICROSCOPY 



chemical identifications, which are the sub- 

 ject here, usually depend upon the forma- 

 tion of crystals by the action of a chemical 

 reagent, which may even pick out the sub- 

 stance in a complex mixture for the analyst. 

 Refractive indices are very seldom used, but 

 optical-cry stallographic properties which 

 can be observed directly, without separating 

 the crystals from the solution in which they 

 are formed, are used, and therefore the 

 polarizing microscope is needed. The two 

 procedures go well together, but they are 

 distinct. Figure 1 illustrates crystalline de- 

 posits examined between crossed nicols; the 

 compound is aminotriazole. Figure lb repre- 

 sents only 0.4 microgram in the deposit. 



Other types of crystallization of a sub- 

 stance without a reagent can be seen in the 

 photographs of r/-, (//-, and (d + dl)-airi- 

 phetamine hydrochloride and of NH4CI (see 

 pages ()7-69). 



It should be noted that optical crystallog- 

 raphy for substances already crystalline 

 focuses upon the exact crystal species pres- 

 ent, determined by such things as the acid 

 with which a basic substance is combined, 

 and even the proportion of molecules of 

 water of crystallization, and sometimes the 

 particular one of two or several polymorphic 

 forms of the same substance, for these things 

 often make major differences in the refrac- 

 tive indices and all the other crystal proper- 



FiG. 1. Aminotriazole as seen with the polarizing microscope, between crossed nicols: 

 (a) deposit from evaporated aqueous solution; (b) deposit of 0.4 microgram. 



22 



