. 197- OPTICAL PROPERTIES OF MINERALS. 281 



rals, as Porcelain-earth, which is a decomposed Feld-spar, 

 and to some compound minerals, as Chalk, where it de- 

 pends upon some particular circumstances in the formation. 



. 197- SERIES IN THE DIFFERENCES OF LUSTRE. 



The gradations in the kinds and in the degrees 

 of lustre in the varieties of one and the same natu- 

 ral-historical species, produce continuous series. 



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 com- 

 pare 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 we are capable of observing any inter- 

 ruption or want of continuity. 



From the succession in these gradations, the above men- 

 tioned series arise, llhombohedral Uuby-blende presents a 

 striking example of a similar series. In some of its varieties 

 the lustre is nearly metallic, in others decidedly adaman- 

 tine. Between these there are a great number of grada- 

 tions of metallic adamantine lustre, by which the two 

 kinds of lustre are so .closely connected, that it is impos- 

 sible to say where the one begins, and where the other ter- 

 minates. 



The series in the gradations of lustre allow of the same 

 application as the series in the varieties of colour mention- 

 ed in , 202. The results of crystallography in respect to 

 simple forms, are exactly confirmed by the occurrences of 

 lustre in single individuals, as to the existence of forms, 

 which, although they appear with a number of faces, not 

 sufficient to limit the space from all sides, yet must be 

 considered as simple forms (. 1G8.). For if we abstract 

 from what is merely accidental, homologous faces entirely 

 .agree as to the kind and to the degrees of intensity of 



