CHEMICAL MICROSCOPY 



sons are available. This goes well beyond about this state of affairs is that it should 



using the tests as merely "confirmatory" of be a challenge and a stimulus to an analyst 



something already known or suspected, but to promote application of the microscope to 



even that is one of their useful functions. any problems of identification that occur in 



The second real disadvantage listed above his own work, 

 for these tests is the limited field of applica- It may be pointed out that these disad- 

 bilitj^ or lack of coverage outside a limited vantages, serious as they are in some cases, 

 field. It has already been pointed out that in no way interfere with the use of micro- 

 the applicability is in fact not nearly so lim- crystal tests by an analyst acquainted with 

 ited as generally supposed, but the disad- them, in: (a) confirmatory tests, where ten- 

 vantage might be stated more accurately tative identification has been made, and 

 that tests have as yet been well worked out comparison with a known sample is possible; 

 only for inorganic ions and the familiar al- (b) in proving the presence of a particular 

 kaloids. The same reagents and procedures compovmd (here, if the compound sought 

 as for alkaloids can be used for a great many turns out not to be present, the test may do 

 other drugs, e.g., antihistamines, and with more than most to show what is present); 

 some changes they can be extended over all (c) in deciding, among a few alternatives, to 

 derivatives of basic nitrogen, and even fur- which one an unknown corresponds: this is 

 ther, but certainly other types of organic often possible by microscopic test-compari- 

 precipitation have not been studied nearly sons with the known alternatives even if 

 enough, and in fact with the many new drugs nothing has been previously published or 

 the coverage is inadequate even for those studied on these particular compounds; (d) 

 giving tests with the traditional reagents. in distinguishing between closely related 



Rather than any danger of making erro- compounds (including distinction of an 



neous identifications, as seems to be feared isomer from the racemate, or even of the 



by many (perhaps rightly in the case of the d- from the ^form, as previously explained); 



totally inexperienced), the bane of the ex- (e) in a larger and larger field of tests as the 



perienced analyst is the finding of an occa- experience of the analyst grows, 

 sional compound, in the line of work, which 



yields beautiful crystals, sometimes with practical norK 



several different reagents, but which, never- As textbooks are nearly non-existent, 



theless, he still cannot identify. This is due some advice on practical work will be at- 



much more to lack of study of the com- tempted. In the limited space it still must 



pounds than to the difficulty of looking up be rather general. 



those already studied, because of lack of First, accustom oneself to the micro- 

 classification. The individual chemist or a scope by using it. Much analytical work will 

 single laboratory cannot possibly keep abreast be helped enormously by preliminary micro- 

 of developments. scopic observation of the material to be 



Both of the disadvantages just discussed examined, either by reflected light (if it is 



are related to general neglect, which also opaque) or by transmitted light. This may, 



has other featm'es. The analyst must often for example, show at the outset whether the 



train himself; he usually cannot obtain material is a mixture or appears to be all one 



much instruction, or even textbooks, in this substance. 



field. At the present time the writer does Then, bring the chemical work down to 



not even know of a good textbook for micro- the level of convenience for microscopic ob- 



crystal tests for alkaloids that is now in servation. A precipitation test can be ob- 



print. The only good thing that can be said served just as well with a drop of solution 



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