CHEMICAL MICROSCOPY 



The sensitivities can be great for both, but oped. They are destructive aUke to the speed, 

 they are usually due to different reactive simplicity, ease, and directness of the micro- 

 radicals or atoms, sometimes both kinds crystal tests, in most cases without any con- 

 present in the same molecule, as with mor- current gain, since the observation of char- 

 phine, and sometimes only the one or the act eristic crystals was already adequate to 

 other present for any known and useful tests, the pmpose of identification. Much of the 

 It would be almost as sensible for a chemist literature on the use of picric, styphnic, and 

 to cut off one hand, as to refuse to use one or picrolonic acids, reinecke salt, etc., is of this 

 the other kind, according to their merits in type, 

 particular cases. A related erroneous idea sometimes en- 



The preeminent radical for color tests is countered is that special measures should be 



probably the phenolic hydroxyl. Even more taken to get the crystals as well-formed crys- 



preeminent for precipitation and micro- tallographically as possible — whereas in fact, 



crystal tests is the basic or partly basic ni- crystals ordinarily forming in characteristic 



trogen atom, no matter whether it is "pri- distortions have far greater value. In most 



mary", "secondary", or "tertiary", or cases even procedures such as warming or 



whether it belongs to a straight chain of adding alcohol are of value only when a 



atoms, or is attached to a benzene ring, or is particular compound is suspected that needs 



itself part of a ring. The chief count eractant special measures to induce crystallization 



to the precipitating effect of basic nitrogen with a much-used reagent, and then a special 



is acidic oxygen, which shows why color reagent is usually better if one can be found 



tests may occasionally "oppose" crystal that will readily give crystals withovit such 



tests, i.e., the change of a molecule to in- measures. 



elude the preeminent radical for color tests Crystals given by a particular organic 



may weaken and lessen the sensitivity of compound may be specific, that is, of a kind 



precipitation tests, especially for relatively given by no other substance. Sometime ob- 



simple compounds. Paradoxically, however, jection is made to calling such tests specific, 



oxygen itself may be basic, in certain cases, but on grounds that would prevent any tests 



and then forms oxonium salts, which are sub- ever being called specific: that not every 



ject to precipitation tests. possible substance has been tried, or even 



While an identification may well be based that some new compound might be discov- 



on several results obtained separately by ered or synthesized sometime, that might be 



different disciplines, it is unfortunate that confused with the old one. To the writer it 



numerous researchers consider the form and seems more reasonable to call a test specific 



obvious characteristics of crystals only as if it is specific, so far as we know or can rea- 



dressing for procedures resulting in micro- sonably foresee, for our knowledge never can 



melting points, refractive indices, or other be infinite. 



numerical determinations. That is, they On the other hand, we should exercise 



make the tests over in the usual pattern of considerable caution in designating a test as 



organic confirmatory tests: formation of a specific. The crystals are, in general, direct 



derivative, isolation of the derivative and compounds of the substances for which the 



often its further purification (e.g., by recrys- tests are made, and therefore in a particular 



tallization), and physical measurement of case they are more or less different from 



some characteristic of the isolated, purified those given by any other substance, just as 



derivative. Such procedures have been pro- each substance is, in itself, finally unique, if 



posed even in the alkaloidal field, where the tests could be carried far enough to take in 



microcrystal tests are already best devel- all its physical and chemical properties. 



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