of Crystallized Bodies. 281 



servable ; they afford an excellent character in ascertaining the 

 forms of the di-prismatic lead-baryte, and in distinguishing 

 them in particular from the forms of the prismatic lead-baryte, 

 or the sulphate of lead, when these minerals are found accom- 

 panying each other, which is often the case, as at Leadhills. 

 Fig. 1 1 represents a very beautiful groupe of three individuals, 

 joined in the manner of Fig. 7, in Mr Allan's cabinet, from 

 that locality. The crystals are very much compressed be- 

 tween the faces I and I. 



Hauy has given a good explanation of a composition simi- 

 lar to Fig. 8, without the faces Zand w, but he has not been so 

 fortunate in the illustration of what he calls the plomb carbonate 

 triple, a composition nearly similar to the one described above, 

 and represented in Fig. 6 ; for he supposes here, that three 

 crystals already formed and complete, are joined, and require 

 exactly 360 degrees of angles to fill up the space in the centre 

 m 9 and thus he actually places the crystals in a perfectly pa- 

 rallel position, in which they would form but one individual ; 

 whereas in fact, it is only necessary to trace the position of the 

 individuals, and leave to the power of crystallization to fill up 

 any vacuities, that may originate in the joining round a cen- 

 tre of several edges, that form a sum short of, or exceeding 

 360°. He is of opinion, also, that, besides the one mentioned 

 in the composition of plomb carbonate hemitrope, there are 

 several other ways in which the crystals are grouped. An 

 accurate observer will always be able to trace the law which 

 requires any two individuals to be joined in such a position, 

 that the axis of revolution is perpendicular upon one of the 

 faces of M. 



The usual crystals of arragonite might be explained from 

 drawings executed for the preceding species, were it not for the 

 difference in the forms which its simple crystals affect. Fig. 

 12 represents one of those discovered a few years ago in the 

 Czinczow mountain, near Bilin, in Bohemia. They are ge- 

 nerally modified by various additional facets, at the place of the 

 solid angles n and n'. The plane of composition is parallel 

 to one of the faces il/, the same as in Fig. 13, where, more- 

 over, the substance of the individuals is continued beyond the 

 face of composition. The last is a Spanish variety. The 14th 



