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.^ 
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.^ 
Fundamental Facts — Optical Crystallography. — In addition 
' 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. 
