Formation of Compound or Twin Crystals. 297 



was referred to a general principle without any accompanying elu- 

 cidation. 



In Fig. 1, amh en represents a vertical section of a Hexahe- 

 dral Prism of Carbonate of Lead or White Lead ore, having its an- 

 gles as follows; a, b, c and o equal to 117° 14' the angle of the 

 primary rhombic prism ; m and n equal to 125° 32' or twice the 

 acute angle of the prism. This form may proceed from the com- 

 pound nucleus represented in the same figure, and is a consequence 

 of the occurrence of adaxal composition on two faces of a molecule, 

 as is shown in the figure, A being united to B and C by its faces 

 about an obtuse angle. This is an occurrence analogous to that 

 which gives rise to the simple twin crystals shown in Fig. 13, 

 PI. L A simple calculation founded on the angles of the circum- 

 scribing rhombs and their situation will prove the angles above given, 

 to be those which would result from such an arrangement of the 

 molecules. The same arrangement is usually given in explana- 

 tions of the formation of this prism, and is in fact the only one which 

 is possible. There can then be no doubt as to its correctness, and 

 only a doubt, if any is possible, as to the existence of the power which 

 is here supposed to unite the molecules. 



When the component prisms of this Hexahedral Prism have their 

 acute lateral edges replaced by a tangent plane, (a very common 

 modification of the Rhombic Prism,) a stellated figure with three 

 rays is formed. The relative situation of these rays to the mole- 

 cule, is shown in Fig. 13 b, PI. L It will be seen there, that the 

 direction of the ray is in the line of the shorter diagonal of the rhomb, 

 and the sides of the ray are parallel to this line. The same is man- 

 ifest in Fig. 1. This stellated form is not of unusual occurrence 

 among the crystals of White Lead ore. 



A Hexahedral Prism of different angles is represented in Fig. 3, 

 and with its pyramidal terminations resembles much a secondary to 

 a Rhombohedron, from which however it is readily distinguished by 

 the inequality of its lateral angles. This form occurs in the same 

 mineral. White Lead ore. It has three alternate angles, (see 

 its horizontal projection in Fig. 2,) a, b, c, each equal to 117° 14' 

 the primary angle, two m and n equal to 121° 23', and another o 

 of 125° 32'. In this instance adaxal composition has first taken 

 place between A and B, after which the molecule B was drawn to 

 its situation by the operation of the opposite poles, which in the fig- 

 ure are represented nearly in contact. In the preceding prism, the 



Vol. XXX— .No. 2. 38 



