376 CHARACTERISTIC. . 846. 



edge in succession; thus, in prismatic Topaz, P = 141 

 7', 101 52', 90 55', &c. This is observed in every spe- 

 cies, where the fundamental form commonly occurs in na- 

 ture, and therefore may be often observed ; or where the 

 combinations do not present any peculiar character, as the 

 hemi- and tetarto-prismatic, or the di-rhombohedral ones, 

 &c. If the fundamental form is seldom observable, its angles 

 are not indicated ; in their stead, however, the characters 

 contain the angles of such derived forms, as commonly are 

 to be met with in nature, as in prismatic Hal-baryte 

 fr = 105 6'; (Pr+co) 3 =77 27'. In every instance re- 

 garding horizontal prisms, that angle is given which is con- 

 tiguous to the axis of the fundamental form, in vertical 

 prisms the angle corresponding to the obtuse terminal edge 

 of the fundamental form. If the combinations of a species 

 possess a particular character, the Characteristic indicates 

 only those dimensions of the fundamental form, which cor- 

 respond to that character, and therefore are immediately 



p 

 observable, as in paratomous Augite-spar, _ = 120 0' ; in 



a like manner in rhombohedral Fluor-haloide, 2 (11) = 

 131 14', 111 20', &c. 



In those forms which assume a hemi-prismatic character, 

 it is very often the case, that the axis of the fundamental 

 form is not perpendicular upon the base (. 98.). The in- 

 clination of the axis takes place in a plane through the 

 axis, and one of the diagonals of the base. This plane bi- 

 sects the angle produced by those faces of the simple forms, 

 which appear in the combination, and which therefore is in- 

 dicated in the designation of the form. Together with 

 the magnitude of this terminal edge, the angle of incli- 

 nation itself is likewise contained in the character. Thus 



p 

 in prismatic Azure-malachite " =117 37', Inclination 



(of the axis), = 2 3 21'" signifies, that those faces of the 

 fundamental form, which necessarily appear together in 

 the combinations, are contiguous to the short diagonal, and 

 meet under angles of 117 37'; and that the axis of P 



