252 ELEMENTARY CHEMICAL MICROSCOPY 



are characterized by a more or less symmetrical black cross if 

 viewed between crossed nicols. 



Very rapid crystallization gives rise to the formation of crys- 

 tals imperfectly developed, the growth generally being most 

 rapid in the direction of the axes or of the boundaries of the 

 facial polygons. The bodies resulting are called skeleton or 

 skeletal crystals. 



Under like conditions of formation, crystalline compounds 

 always separate not only in the same crystal system, but will 

 assume each time the same geometrical form; this character- 

 istic form is called the habit of the compound and upon this 

 property microchemical methods of analysis are based. Pro- 

 viding we can control the conditions influencing the formation 

 and the separation of a crystalline compound upon a glass object 

 slide, we may be reasonably certain that in every experiment 

 tried not only will we obtain exactly similar crystals but also 

 that the great majority of the crystals will always lie upon the 

 slide in a similar position. 1 



Crystals in the course of their growth invariably occlude mother 

 liquor and futhermore will be found to contain inclusions of air 

 or gases, and by virtue of adsorption or solid solution phenomena 

 will contain foreign matter which may be present. Theoretically, 

 the separation of an absolutely pure crystal of a salt consisting 

 of a single solid substance alone is an impossibility when dealing 

 with a mixture. 



When the foreign matter present is such that the adsorptive 

 power of the salt for it is great, not only may the crystal habit be 

 profoundly changed but the color and the characteristic proper- 

 ties of the salt may also be altered. It is possible to thus obtain, 

 by the means of vegetable and aniline dyes, colored crystals 

 from colorless inorganic salts. 2 



Fundamental Facts Optical Crystallography. In addition 



1 E. von Fedorov has recently compiled an elaborate set of tables in the Zeit. 

 Kryst. Min., 60, 513, whereby it is possible to identify a compound through its 

 crystallographic habit and properties. It is suggested that this mode of analysis 

 be called Crystallo-Chemical Analysis. 



See also Orelkin and Pigulevski, J. Russ. Phys. Chem. 46, 227. 



2 See Gaubert, Recherches recentes sur les facies des cristaux. Paris, 1911. 



