282 TERMINOLOGY. . 198. 199. 



their lustre ; and, vice versa, such faces which do not agree 

 with each other in the same respects, are not homologous, 

 or they do not belong to one and the same simple form. 

 This is equally applicable to faces of crystallisation, and 

 to faces of cleavage, as is shewn by numerous examples in 

 prismatoidal Gypsum-haloide, in several species of the 

 order Mica, in several species of the genus Kouphone-spar, 

 &c. Pearly lustre is the most remarkable among the dif- 

 ferent kinds, since, in a high state of perfection, it appears 

 in simple minerals only upon single faces of crystallisation, 

 as well as of cleavage. Such faces, therefore, are parallel 

 either to the axis, or to the base of the fundamental form 

 of the species. A single face which possesses a distinc t 

 pearly lustre, if it is a face of cleavage, is also termed 

 Eminent. The pearly lustre of compound minerals very 

 often is a consequence of this composition. 



. 198. COLOUR, PROPERLY SO CALLED, AND STREAK. 



We have to distinguish between the colour of 

 the entire mineral and that of its powder. The 

 first is the Colour of the mineral, properly so call- 

 ed, while the second has been designated by the 

 name of the Streak. 



. 199. DIVISION OF COLOURS. 



The colours have been divided into metallic and 

 into non-metallic colours. 



This division is not rigorously correct, because the dif- 

 ference does not so much lie in the colours themselves, as 

 in the kinds of lustre joined to them. It is, however, very 

 useful, since it separates what is as yet indispensable, from 

 what is merely useful in the process of discriminating mi- 

 nerals. 



For the sake of a better distinction of the colours, eight 

 -principal colours have been assumed by the celebrated 



