X PREFACE. 



geneous in its mechanical composition and at the same time 

 destitute of a fixed chemical constitution. 



The general order observed in the particulars of the de- 

 scriptions is that adopted by MOHS, whose language in re- 

 lation to a number of the properties .has been employed, 

 verbatim. The description of the regular forms, however, 

 is quite different and requires some explanation. In many 

 instances, all the observed forms are represented ; in oth- 

 ers, where the number was too great to admit of this, such 

 a selection has been made as appeared best adapted to give 

 a correct idea of the entire series of modifications within 

 the species. The locality is sometimes subjoined, when 

 the form represented is not common, or when it is known 

 to occur at a particular spot in unusual perfection. 



The primary forms of BROOKE have been adhered to, 

 more for the reason that they are still in such common use 

 in English works and from their expressing in general the 

 cleavage forms with correctness, than because they are in 

 every instance the most simple solids from which the sec- 

 ondaries are capable of being derived. In the last men- 

 tioned view, they would certainly admit of a very important 

 reduction. The tetrahedron, cube, regular octahedron 

 and rhombic dodecahedron might form a single system, 

 the right square prism and the octahedron with a square 

 base, another, the right rectangular prism and the octahe 

 dron with a rectangular base, a third, and the right rhom- 

 bic prism and the octahedron with a rhombic base, a fourth 

 system. 



For the sake of easy intelligibility also, the angles and 

 faces of the crystals are described without adopting an al- 



