106 TERMINOLOGY. 



degree of lustre is observable in the columnar composition, (often 

 called fibrous fracture,) and in other compound minerals in which 

 the composition is no longer observable, as in the varieties of Quartz 

 called Hornstone, Flint, and Chalcedony. This degree of lustre may 

 generally be taken as a sign of a compound mineral, the individuals 

 of which are so very small, as nearly to disappear. It is produced 

 by the reflection of light, from every one of the impalpable com- 

 ponent parts. 



Dull, possesses no lustre at all. This perfect absence of lustre is 

 almost entirely confined to decomposed minerals, as in Kaolin. 



. 89. SERIES IN THE DIFFERENCES OF LUSTRE. 



In general, neither the kinds nor the degrees of lustre 

 admit of rigorous limits. It is necessary to determine them 

 in some particularly distinct examples, and to compare with 

 them such as are less distinct. 



If there occur several kinds or degrees of lustre in the varieties 

 of a species, these will be in an uninterrupted connexion, and they 

 will pass insensibly into one another, so that in no place are we ca- 

 pable of observing any interruption or want of continuity. Out of 

 the succession in these gradations, the series in question arises. 



If we make abstraction of what is merely accidental, similar fa- 

 ces in single individuals agree as to the kind and degree of in- 

 tensity of- their lustre; and, on the contrary, such faces as are riot 

 similar, disagree in this respect. This is equally the case as relates 

 to faces of crystallization, and to faces of cleavage, as is shown by 

 examples from Gypsum, Mica, and Stilbite. Pearly lustre is the 

 most remarkable among the different kinds; since, in a high state of 

 perfection, it appears in simple minerals only upon single faces 

 of crystallization as well as of cleavage : example, Heulandite. 



. 90. DIVISION OF COLORS. 



The colors are divisible into two series: 1. The metallic 

 colors ; 2. The non-metallic colors. 



For the better distinction of colors, Werner was led to assume 

 eight principal colors as the foundation of all the others. These 



