102 ON THE TABLES OF MODIFICATIONS. 



The figure of the octahedron with a rectangular 

 base, has the greater edge of its base horizontal. 



The figures of the right rhombic prism, and of the 

 octahedron with a rhombic base, have their greater 

 diagonals horizontal. 



The figure of the right oblique-angled prism, has 

 its greater diagonal horizontal, and the greater lateral 

 plane of the prism standing opposite the right hand of 

 the spectator. 



Thus where the angles of the base of a primary 

 form are right angles, four of its terminal edges, as 

 seen in the figures, are horizontal. But where the 

 angles of the base are not right angles, the figure is 

 drawn with its greater diagonals horizontal, and all its 

 terminal edges are oblique lines. 



1 conceive that an advantage will attend the placing 

 the figures of crystals of different minerals, belonging 

 to the same class of primary forms, always in the 

 same position. The crystals of different substances 

 may be then more easily compared with each other, 

 and their peculiar characters be more readily ob- 

 served. Euclase, and sulphate of lime, are right 

 oblique-angled prisms; but the figures of euclase 

 which accompany the Abbe Haiiy's description of that 

 substance, are made to rest on one of the lateral 

 planes of the primary form, as it is exhibited in p. JO, 

 while his primary form of sulphate of lime is made to 

 rest on its base as that figure does. 



On each of the primary forms, it will be observed, 

 that certain letters are placed, which are intended to 

 designate the angles, edges, and planes, of crystals, 

 and to denote their similarity or dissimilarity. 



We are indebted to the Abbe Haiiy for the adapta- 

 tion of letters to these purposes, and in a future 

 section the manner will be explained in which they 



