MICRO-ANALYSIS. 817 



of the Hemoglobins." * By special means individual crystals 

 of the hemoglobins were obtained and by purely crystallographic 

 methods, including a study of the forms and optical properties 

 of such crystals, the hemoglobins of the 200 species of animals 

 studied were differentiated in a manner that could not have been 

 accomplished by chemical analysis or other methods of procedure. 



A careful study of much that has been written, and especially 

 of the illustrations that have been made, of micro-crystals in 

 plants and drugs, shows that erroneous conclusions may be easily 

 drawn from the general appearance of crystalline precipitates or 

 aggregates of crystals that are formed. For instance, Vogl 

 has shown that the sphero-crystals, found in the glandular hairs 

 of Mentha piperita and considered by some to be menthol, are 

 found in leaves of many of the Labiatse. (See also Figs. 158, 

 175, 272.) Again very many substances produce aggregate 

 groups which closely resemble each other, as of citric acid, cocaine 

 hydrochloride, etc. 



In regard to the value of the crystallographic method we 

 quote the following paragraph from Brown {loc. eif.) : "When 

 a chemical compound solidifies from fusion, solution or vapor 

 under conditions which are favorable to the development of 

 individuals, its particles tend to arrange themselves in regular 

 order, so that a definite structure is produced. The external 

 form of the individuals is also regular, being bounded by planes 

 in definite relation to each other so that polyhedral solids are 

 produced which are called crystals. The regular arrange- 

 ment of the atoms among themselves, and of the molecules 

 which they build up, is so characteristic of substances of definite 

 composition that the crystalline condition of dead matter is the 

 normal condition. Differences in chemical constitution are ac- 

 companied by differences of physical structure, and the crystal- 

 lographic test of differences of chemical constitution is recognized 

 as the most delicate test of such differences." 



It is apparent that apart from their solubility, color reactions, 

 behavior towards reagents, etc., the substances with which we 

 are dealing should be prepared in such a manner that isolated 



* Published by the Carnegie Institution of Washington, 1909. 



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