. 159- OF THE IMPERFECTIONS OF CRYSTALS. 215 



while the common goniometer requires crystals of a larger 

 size, if it shall be applicable at all, and these are but rarely 

 found in the necessary perfection. 



The reflective goniometer must necessarily be employed 

 in those accurate and desultory observations, upon which 

 are founded the determinations of the angles of the funda- 

 mental form, and consequently also the dimensions of a 

 natural-historical species ; the common goniometer, how- 

 ever, will always be found sufficiently accurate, if our ob- 

 ject be only to discriminate individuals, or to find out the 

 place of a given one in the system, by the assistance of the 

 Characteristic. 



Notwithstanding all the variability in the size and in 

 the figure of the faces depending upon it, both of which 

 may be referred to the imperfect formation of the crystals ; 

 yet the situation of those faces towards each other will be 

 always found constant. The faces of forms, both simple 

 and compound, constantly intersect each other at the same 

 angles which they would produce if the form had arrived 

 at the highest possible degree of perfection, which depends 

 upon the exact equality and similarity of the homologous 

 faces. The magnitude of the angles is constant. This re- 

 markable fact has first been ascertained and demonstrated 

 by the celebrated ROME DE L'!SLE ; it is the basis upon 

 which is founded the possibility of applying crystallography 

 to the mineral kingdom. Doubts have been raised against 

 the correctness of that law, derived from an apparent transi- 

 tion of certain crystalline forms into others, by continual 

 changes in the magnitude of the angles: these doubts, 

 however, immediately disappear, if we consider the crystals 

 in their greatest geometrical perfection, and not affected 

 by any of those irregularities to which they are subject. 

 Hence we may infer, that the crystallisation of minerals, 

 from the simplicity and constancy of its laws, under the 

 appearance of the greatest variability, deserves on that ac- 

 count to be called the most remarkable of those phenomena 

 which inorganic nature presents to the observer. 



