MICROCHEMISTRY. 143 
the various faces of a crystal vary widely with the supply 
of material in the solution from which they were deposited. 
The interfacial angles between corresponding faces and 
the facial angles between corresponding edges are, how-’ 
ever, always constant for a given substance. This law, 
known as the Law of Steno, forms the basis for determina- 
tive crystallography, and makes it possible to distinguish 
the crystals of such compounds as are not isomorphous. 
Besides the general crystalline form, there are numerous 
special points to be noted in the study of a crystal. Its 
habitus (the size and proportions due to the conditions 
attending its deposition) should be noticed, as well as 
its color and the approximate refractive index, indicated 
by the definition of the edges either in air or in some 
denser mounting medium. The effect upon polarized 
light, which will be discussed in Chapter XII, is often 
of great importance. Various peculiar phenomena may 
appear, such as hemimorphism (differences in the opposite 
ends of a crystal), twinning (the production of double 
crystals), skeleton crystals (parallel or symmetrical 
aggregates of smaller crystals), trichites (hair-like crystals, 
often more or less twisted), and spherulites (radially 
fibrous spherical bodies). Sodium chloride crystallized 
rapidly from a solution thickened with mucilage exhibits 
beautiful skeleton crystals; and long, curved trichites 
appear in a mixture of the chlorides of chromium and 
mercury. 
The change in crystalline form of certain compound 
salts furnishes an interesting subject for study under the 
microscope. If ferrous chloride is allowed to crystallize 
