318 THE MICROSCOPE AND ITS REVELATIONS. 



CHAPTEK XX. 

 CRYSTALLIZATION. POLARIZATION. MOLECULAR COALESCENCE. 



710. ALTHOUGH by far the most numerous and most important appli- 

 cations 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 combined. The natural forms of Inorganic 

 substances, when in any way symmetrical, are so in virture of that pecu- 

 liar 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 in- 

 crusted with an endless variety of secondary formations of the same kind, 

 some consisting of thin laminse 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 00 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 microscopic crystallizations sometimes present 

 the same curious phenomenon (Fig. 495). 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: 



Antimony, sulphuret Iron, ilvaite or Elba-ore 



Asbestos pyrites (sulphuret) 



Aventurine Lapis lazuli 



Ditto, artificial Lead, oxide (minium) 



Copper, native sulphuret (galena) 



arseniate Silver, crystallized 



malachite-ore Tin, crystallized 



peacock-ore oxide 



pyrites (sulphuret) sulphuret 



ruby-ore Zinc, crystallized. 



1 See Mr. Glaisher's Memoir on ' Snow-Crystals in 1855,' with numerous beau- 

 iful figures, in " Quart. Journ. of Microsc. Sci ," Vol. iii. (1855), p. 179. 



