FORMS OF MICROCRYSTALS 



33-42 (1955); "Microchemical Tests for the 

 Identification of Alkaloids," /. Pharmacy 

 and Pharmacology, 7, 255-62 (1955). 

 15. Clarke, E. G. C. (a) "Microchemical identi- 

 fication of Sugars as Osazones", J. Phys- 

 iology, 135, 28-9 P, from "Proceedings of 

 the Physiological Society," December 1956. 



(b) In /. Pharmacy and Pharmacology, 

 "Microchemical Identification": Local Ana- 

 esthetics, 8, 202-6 (1956); Some Less com- 

 mon Alkaloids, 9, 187-92 (1957); Antihista- 

 mines, 9, 752-8 (1957); Antimalarials, 10, 

 194-6 (1958). Atropine-Like Drugs, 11, 

 629-36 (1959). "Microchemical Differentia- 

 tion between Optical Isomers of N-Methyl- 

 morphinan Analgesics," 10, 642-4 (1958). 



(c) "Microchemical identification of some 

 modern analgesics," Bulletin on Narcotics, 

 11, No. 1, 27-44 (1959). 



Charles C. Fulton 



FORMS OF MICROCRYSTALS 



Descriptions and classifications of micro- 

 chemical crystal forms have scarcely any 

 relation to orthodox crystallography. What 

 is needed is not a deduction of the crystal 

 system — even if that were generally possible, 

 which it is not — but simply a straightforward 

 way of describing and classifying the forms 

 just as they are seen as a result of the chemi- 

 cal test. 



In the comprehensive study and classifi- 

 cation of microcrystals, along with /orm goes 

 size, and the latter is measurable. In the fol- 

 lowing classification of forms, however, the 

 concern is not with the actual measured size, 

 but only with considering, in relation to 

 form, how many different measurements, of 

 value to a description or classification, might 

 be made on a particular type of crystal. 



Crystals exist, of com'se, in three dimen- 

 sions, but may extend significantly only in 

 one direction, length, if they are fine needles, 

 —or only in two, length and breadth, if 

 they are plates or blades of negligible thick- 

 ness. In the latter case, moreover, they may 

 lie flat, especially if under a coverslip; so 

 that such crystals, certainly as we see them, 

 extend simply in two dimensions. 



The number of dimen.sions subject to use- 

 ful measurement will not (in general) be 

 greater than the number of dimensions of ex- 

 tension, but may be smaller. In the case of a 

 regular hexagonal plate, for example, only 

 one measurement is needed to complete the 

 description (so far as form and size go) — no 

 matter whether we make it as the length of 

 a side, or as a radius, or as the diameter from 

 one vertex to the opposite one. 



The writer uses the term "grain" for crys- 

 tals which resemble the familiar grains of 

 sand, salt, sugar, etc., as they appear under 

 moderate magnification. The three dimen- 

 sions of "extension" are essentially equal, so 

 that only one measurement is needed (the 

 diameter) to complete a description in a 

 particular case. 



These dimensional concepts give six 

 classes; but there are two distinct kinds of 

 crystals extending in three dimensions, each 

 requiring two different measurements. They 

 may best be taken as two separate classes 

 (see table). It is also convenient to separate 

 elongate or directional plates from blades. 

 Together with a zero class for crystals of no 

 significant dimensions (appearing as mere 

 specks with the magnification usually used), 

 this arrangement therefore provides nine 

 classes in all (0-8) for the simple crystal 

 forms. Distortions, contortions, and skele- 

 tons, as well as aggregates, should be referred 

 to the corresponding sunple forms. 



Skeletonized crystals are here classed with 

 the forms from which they are derived 

 (which may occur along with them), rather 

 than with the simple forms they may finally 

 resemble in their parts. For example, some 

 crystals vary from square plates to crosses. 

 The flat, thin cross should still be assigned 

 to the same class as regular plates although 

 it might be described as having arms of 

 blades. 



Skeletonized forms can often be distin- 

 guished from aggregates by the birefring- 

 ence. If a complex form extinguishes and 

 brightens as a whole, it usually is basically 



37 



