. 256. PHYSIOGRAPHY. 9 



bohedral Lime-haloide so very readily yields to cleavage, 

 that in most cases it is attended with considerable difficulty 

 to perceive the faces of fracture, which are conchoidal. 



The physical quali ty of the surface of crystals is far more 

 important than fracture, since it is in close connexion with 

 the crystalline forms themselves. Also these faces are ex- 

 pressed by crystallographic signs, because they cannot be 

 indicated with greater brevity or precision. Several of the 

 faces of crystallisation of rhombohedral Lime-haloide are 

 very often found to be streaked ; but these striae, with a few 

 trifling exceptions, have a constant direction, being almost 

 in every instance parallel to the intersection of the striated 

 faces with the faces of the rhoinbohedron R. 



The characters depending upon the presence of light, 

 contribute very much to enliven the image or representa- 

 tion of the species. The kinds of lustre must everywhere 

 be mentioned ; and if there should be found a difference as 

 to its occurrence upon different faces, also the direction 

 must be given, in which the different kinds of lustre may 

 be observed. In rhombohedral Lime-haloide, the lustre is 

 in general vitreous, only the faces of R CD sometimes pre- 

 sent pearly lustre. The latter kind of lustre sometimes 

 also extends to compound varieties, whose faces of compo- 

 sition correspond to those of II cs, as in the well known 

 example of Slate-spar. Of the degrees of lustre, it is suffi- 

 cient to mention the limits. 



The series of colours in particular might be very useful 

 in enlivening the collective descriptions, if they could be 

 expressed with the same conciseness as those of the crys- 

 talline forms. But in order to indicate the series, it is in- 

 dispensable to mention all its single members ; and I have 

 therefore thought more convenient, for the sake both of 

 precision and brevity, merely to give an outline of these 

 series, by indicating their principal points, or some of their 

 more peculiar properties. This mode of treating the sub- 

 ject does by no means injure the use of the series of co- 

 lours, nor diminish their importance in the determination 

 of the species. It is the white colour, which, in rhombohe- 



