IMPERFECTIONS OF CRYSTALS. 



57 



Thus, hexagonal crystals of Beryl frequently have their alternate 

 lateral faces so enlarged as to give them the appearance of triangular 

 prisms. The faces of a cubical crystal of Galena very often are not 

 squares. Dodecahedral crystals of Garnet are sometimes elongated 

 in the direction of one of the lesser axes, by which means it becomes 

 apparently a regular six sided prism, surmounted by trihedral sum- 

 mits, &c. 



The same thing takes place with regard to the modifications of 

 simple forms. Some of the faces of the modification are irregularly 

 increased, whilst corresponding faces belonging to the same forms 

 are diminished, till they almost or wholly disappear. This remark 

 is exemplified in the crystals of Quartz. 



In addition to the above irregularity, we have, in the crystals of a 

 few species, slight curvatures in the faces. If these curvatures take 

 place in simple forms, it generally affects all the faces at once, as in 

 the crystals of Diamond and Fluor, If it take place upon modify- 

 ing planes, it is confined to those which are similar, as m certain 

 crystals of Gypsum. 



Notwithstanding all the irregularities arising out of the dispropor- 

 tionate extension of similar faces, the inclination of these faces to each 

 other is invariably constant, and precisely the same as though their 

 dimensions were exactly similar, and the form were possessed of the 

 highest degree of perfection. For example, in Figures 132 and 133, 

 representing crystals of Quartz, any two planes which may be se- 

 lected in Fig. 132 incline to each other under the same angle as the? 

 two similarly situated planes in Fig. 133, 



Fig. 132. 



Fig. 133. 



This remarkable fact was first ascertained and demonstrated by 

 Rom6 de PIsle, and is at the foundation of the application of crys- 

 tallography to the discrimination of minerals. 



