MICROCHEMISTRY. I43 



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 e'dges 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 sphseiulites (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 



