768 



CHAPTER XX. 



INORGANIC OR MINERAL KINGDOM. — POLARIZATION. 



597. 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 colours, or for both combined. 

 The natural forms of Inorganic substances, when in any way sym- 

 metrical, are so in virtue of that peculiar arrangement of their 

 particles which is termed Crystallization; and each substance 

 which crystallizes at all, does so after a certain type or plan, — the 

 identity or difference of these types furnishing characters 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 encrusted 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 66° or 120°. Beautiful arborescent 

 forms are not unfrequently produced by the peculiar mode of aggre- 

 gation of individual crystals : of this we have often an example on 

 a large scale on a frosted window ; but mici'oscopic crystallizations 

 sometimes present the same curious phenomenon (Fig. 408). — In 

 the following list are enumerated some of the most interesting 



* See Mr. Glaislier's Memoir on ' Snow-Crystals in 1855,' with numerous 

 beautiful figures, in "Quart. Journ. of Microsc. Science," Vol. iii. (1855), 



p. ivy. 



