CHAPTER XX. 



INORGANIC OR MINERAL KINGDOM. POLARIZATION. 



454. ALTHOUGH by far the most numerous and most important 

 applications of the Microscope, are those by which the structure 

 and actions of Organized beings are made known to us, yet there 

 are many Mineral substances which constitute both interesting 

 and beautiful objects ; being remarkable either for the elegance 

 of their forms, or for the beauty of their colors, or for both com- 

 bined.* The natural forms of inorganic substances, when in any 

 way symmetrical, are so in virtue of that peculiar arrangement 

 of their particles which is termed crystallization ; and each sub- 

 stance which crystallizes at all, does so after a certain type or 

 plan, the identity or difference of these types furnishing cha- 

 racters of primary value to the Mineralogist. It does not follow, 

 however, that the form of the crystal shall be constantly the 

 same for each substance ; on the contrary, the same plan of 

 crystallization may exhibit itself under a great variety of forms ; 

 and the study of these, in such minute crystals as are appropriate 

 subjects for observation by the microscope, is not only a very 

 interesting application of its powers, but is capable of affording 

 some valuable hints to the designer. This is particularly the 

 case with crystals of Snow, which belong to the "hexagonal 

 system," the basis of every figure being a hexagon of six rays ; 

 for these rays " become incrusted with an endless variety of 

 secondary formations of the same kind, some consisting of thin 

 laminae alone, others of solid but translucent prisms heaped one 

 upon another, and others gorgeously combining laminae and 

 prisms in the richest profusion ;" ! the angles by which these 

 figures are bounded, being invariably 60 or 120. Beautiful 

 arborescent forms are not unfrequently produced by the peculiar 

 mode of aggregation of individual crystals ; of this we have often 

 an example on a large scale on a frosted window ; but micro- 

 scopic crystallizations sometimes present the same curious phe- 

 nomenon (Fig. 345). In the following list are enumerated some 

 of the most interesting natural specimens, which the Mineral 

 kingdom affords as microscopic objects ; these should be viewed 

 by reflected light, under a very low power : 



1 See Mr. Glaisher's Memoir on "Snow-Crystals in 1855," with a number of beauti- 

 ful figures, in " Q.uart. Journ. of Micros. Sci." vol. iii, p. HO. 



