CHEMICAL MICROCRYSTAL IDENTIFICATIONS 



distinguish an individual compound even The science of making chemical identifica- 

 from those others that are closely related, tions is important in forensic chemistry (cf. 

 Likewise, a chemical reaction may be general Forensic Microscopy, p. 338), especially in law 

 for a whole class of compounds, and yet we enforcement, because many legal cases in- 

 may be able to distinguish particular results, volve questions of identity rather than 

 For example, a color reaction of phenols may quantity, as regards narcotics, poisons, po- 

 give different colors with different phenols, tent drugs, adulterants, contaminants, sub- 

 The value of the microscope is to see dif- stitutes, and so on, and such cases require 

 ferent results given by different substances exacting proof of the identity of the sub- 

 even in the same general precipitation reac- stances concerned. The uses are more gen- 

 tion. eral, however, for questions of chemical 



Chemical tests are mainly divided into identity which can be answered in similar 



two kinds — color tests and precipitation ways may arise in any field involving chemi- 



tests. Now the ordinary spectrophotometer cal substances. Usually too little attention 



(cf. Absorption Spectroscopy — ^Visible and is paid to the possibilities of microscopic 



Ultraviolet, in "Encyclopedia of Spectros- chemistry, which may provide tests for very 



copy") may be regarded as giving not only a minute quantities, or such firm assurance of 



means of studying what might be called the identity as to be the preferred method, when 



"colorimetric" properties of a substance it- it can be used by anyone acquainted with 



self, but also, when a reagent is used, an it, regardless of the amount of substance 



enormous extension of the color tests of available for tests. 



chemistry. Not only are comparisons and Microcrystal tests are exceptionally good 



colorimetric readings made better, in the for court purposes, because they are as sim- 



visible range, but also they are extended pie and direct as tests can be. In many cases, 



deep into the ultraviolet, where nothing just looking at the crystals under the micro- 



at all could be seen with the unaided eye. scope enables the analyst to make the identi- 



In a somewhat analogous way, the micro- fication, and the things looked for are the 



scope enormously extends not merely the characteristics of an actual, visible com- 



study of the form and crystal properties of pound of the substance to be identified. It 



a substance itself, but also the ordinary seems to be pretty generally realized that 



precipitation tests of chemistry, without simple, direct tests are the best for forensic 



changing their essential character as chemi- purposes. What sometimes seems to be over- 



cal reactions. Not only are details of the looked or not realized as it should be, is that 



precipitate, when it is crystalline, seen bet- they are also best for chemical purposes, 



ter, and down to a smaller size, but with the In these microcrystal tests the polarizing 



polarizing microscope the observation of microscope should by all means be used, and 



crystal characteristics is extended to things the relation to optical crystallography is, of 



of an entirely different nature from those course, close. Now optical crystallography, 



that can be seen with the unaided eye by for which, in the field of mineralogy, the 



ordinary light. Thus microscopic identifica- polarizing microscope was developed, is of- 



tions by crystal forms and properties are at ten an ideal means of identifjdng the crystal 



least potentially capable of extension from species, for any substance that is already 



substances which are already crystalline to present in microscopically sizable crystals 



all substances that give precipitation reac- or fragments of crystals, and in fair pro- 



tions in ordinary chemistry, or indeed to portion in the material examined; and de- 



those that give any kind of crystal-forming termination of refractive indices plays a 



reaction. large part. On the other hand, microscopic 



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