CHEMICAL MICROSCOPY 



Fig. 5. Crystals produced by Cinchonine with 

 lodine-KI reagent C-3 or a similar reagent. 



with microcrystal tests, because the crystals 

 are not some that come to the analyst al- 

 ready formed mider unknown conditions, 

 but are formed under test conditions, and 

 comparisons should be made with a known 

 sample under as nearly as possible the same 

 conditions. 



Secondly, even if one crystal type was un- 

 expectedly changed into another which was 

 unknown to the analyst, this would pre- 

 sumably result in a failure to make the 

 identification, but not in an erroneous identi- 

 fication. There is probably no way of testing 

 known that may not sometimes fail to give a 

 recognizable result because of some kind of 

 interference with the usual test conditions. 

 On the other hand, if both kinds of crystals 

 are known, even if the reason for one chang- 

 ing into the other is not, the test is still good. 



Finally, the objection is especially strange 

 because in most cases, having two or more 

 types of test-crystals is a positive advantage. 

 If we can learn the reason for the change — 

 and this is usually not very hard to do, and 

 is known in many cases — then we have two 

 tests instead of one, which together often 

 provide enough evidence for identification in 

 themselves. Or, the two kinds of crystals 

 may occur in the same test, in which case 



both types together characterize the sub- 

 stance, and the single test is all the more 

 likely to be specific or very highly character- 

 istic. 



One crystal type is occasionally known to 

 be changed to another by temperature or 

 by stirring. However, the most common 

 cause is a different concentration of the sub- 

 stance, especially relative to the precipitat- 

 ing agent in the test -drop. Particular advan- 

 tage is taken of this in the new tests made 

 by addition of a strongly acid reagent to a 

 very little of the dry substance tested. A 

 cover-glass is applied and this prevents the 

 tested substance from diffusing equally 

 through the test-drop. Thus the precipita- 

 tion and crystallization take place at differ- 

 ent concentrations in each single test even 

 with only a minute amount of the substance. 

 Therefore when a substance gives two or 

 more completely different types of crystals 

 at different concentrations, they are nearly 

 always obtained in each single test, so that 

 such tests are generally highly characteristic 

 and may comparatively often be of specific 

 rank. 



For example, in one such test with an 

 iodine-KI reagent in HCI-H3PO4 , morphine 

 gives four kinds of crystals (Fig. 6a), de- 

 pending upon concentration: black needles, 



Fig. 6a. Crystals given by extracted Morphine 

 with Iodine-KI reagent M-2. 



28 



