of Crystallized Bodies. 279 



considered as modifications of the regular six-sided prism ; a 

 few are yet enumerated as such by very able mineralogists. 

 The correct determinations introduced by modern authors af- 

 forded them the means of more accurately circumscribing the 

 species themselves, — a result which has so frequently rewarded 

 the labours of those who study the geometrical properties of 

 substances. The general appearance of the compound groupes 

 is so much alike in all these species, that from having more 

 circumstantially explained one of them, the rest will be perfect- 

 ly intelligible, at least if we add those observations which are 

 rendered necessary by any occurring peculiarities of the crys- 

 tallization. 



To this end we select the di-prismatic lead-baryte, or the 

 carbonate of lead, and among the forms of the individuals 

 which occur in the composition, that of Fig. 1. Plate VI. Fig. 

 2. being the projection of it on a plane perpendicular to the 

 faces M and I. The crystallographic signs, after the method 

 of Mohs, are Pr (If), (Pr+ oc) 3 (s) and Pr+ oc (I). To the 

 projection, Fig. 2, everything here shall be referred which 

 relates to the explanation of the regular compositions. The 

 faces of composition in this species are parallel to the faces 

 of Pr (M), a horizontal prism of 117° 13'. The result of a 

 regular composition of two individuals is represented in Fig. 3. 

 The faces, ,M and t M' are parallel, M is inclined on M' at an 

 angle of 125° 34/, and I on I' at 117° 13 7 . This last angle is 

 equal to the angle of the prism itself, and follows of necessity, 

 the angle a being = b, and c = d, b and d being each of them 

 equal to one-half the angle of the prism, which is a general re- 

 sult, whatever may be the value of it in degrees. 



As in other systems, the composition is often repeated in 

 parallel layers, particularly near the plane of junction, as in 

 Fig. 4. If the substance of the individuals does not terminate 

 at this face, but is continued beyond it, the groupe assumes in 

 its transverse section a cruciform appearance, similar to Fig. 5. 

 Beside the face of composition no, which is parallel to the 

 faces My M, another face pq is formed perpendicularly upon 

 it, which, therefore, is also perpendicular upon M and 

 M, and when the re-entering angles at p and q are filled up 

 by the increase of the individuals, it is often difficult to as- 



