1094 



CHAPTER XXIY 



MICROCRYSTALLISATION. OPTICAL PROPERTIES OF CRYSTALS. 

 MOLECULAR COALESCENCE. MICRO-CHEMICAL ANALYSIS. 



ALTHOUGH by far the most numerous and most important applica- 

 tions of the microscope were formerly those by which the structure 

 and actions of organised beings are made known to us, yet the in- 

 creased attention which has been paid during recent years to tin* 

 use of the microscope in elucidating the internal structure of 

 crystalline substances, whether of natural or artificial origin, lias 

 made this instrument as indispensable to the crystallographer ;md 

 the mineralogist as it formerly was to the physiologist. Solid sub- 

 stances are almost invariably found in nature or obtained as labora- 

 tory products in the form of individual fragments, each bounded by 

 plane surfaces which are inclined at such angles that the whole 

 figure is possessed of a greater or lesser .degree of geometrical 

 symmetry. Such solid bodies are termed crystals, and, although 

 formerly the regularity of external shape constituted the only avail- 

 able means of recognising them, it is now demonstrated that the 

 external form is only the result of the so-called homogeneous 

 internal structure of the crystal. This homogeneity of structure 

 consists in the arrangement of the smallest characteristic particles 

 or units of the structure being the same about every unit of the 

 structure. The different kinds of possible homogeneous arraniiv 

 ments of points in space have been investigated by Bravais, Sohncke, 

 and others, 1 and on classifying them according to their symmetry 

 they fall into thirty-two classes identical with the thirty-two known 

 crystalline systems. These thirty-two types of structure differ in 

 their symmetry, and this difference is expressed in the symmetry of 

 the external form ; the external form, however, is very liable to 

 distortion, in consequence of a lack of uniformity in the conditions 

 prevailing during the growth of the crystal, and so is at best but an 

 untrustworthy guide to the symmetry of the internal structure. The 

 optical properties of the solid structure, also themselves expressions 

 of the symmetry, and consequently of the crystalline system, are 

 not disturbed by casual influences to nearly so great an extent as is 

 the regular external form ; the symmetrical variation of the optical 

 properties of crystalline structures in accordance with the symmetry 



1 See A. Sclioenflies, Krystallsysteme und KrystaUstntctiir, Leipzig, 1891. 



