84 TERMINOLOGY. 



de Lisle, a name at present but little employed, in consequence of 

 its more general application as the denomination of a species in 

 mineralogy. 



This kind of structure will become apparent from a careful atten- 

 tion to the following examples. 



Fig. 136. 



Fig. 137. 



Fig. 138. 



If, in Fig. 136, the halves of a regular Octahedron, (obtained by 

 placing that solid upon one of its faces and dividing it, in this posi- 

 tion, by a horizontal section, equidistant from the upper and lower 

 parallel planes,) be applied to each other, parallel to the face of sec- 

 tion m n o p q r, Fig. 137, and then made to revolve upon an axis 

 perpendicular to the plane of junction through 60, or one sixth of 

 a circle, a twin crystal, Fig. 138, will be formed. Or we may sup- 

 pose that the crystals instead of being thus bisected, and only the 

 half of each made use of, were applied so as mutually to penetrate 

 each other, and that the revolution, above described, took place in 

 the compound crystal where the sectional planes m n o p q r, of 

 the two crystals, coalesce. The compound crystal possesses angles 

 not to be met with in either of the forms which are supposed to 

 give rise to it; as may be seen in the meeting of the two planes, 

 c m 1 r f andfop, Fig. 138, which form what is called a re-entering 

 angle; the angles also at which the two planes, a c r' q' and bf on 

 incline, are different from that at which any two faces meet in the 

 regular Octahedron. This is called a salient angle in the pres- 

 ent form. This variety of twin crystal, it will be observed from 

 an inspection of Fig. 138, has three re-entering and three salient 

 angles, which are all situated in an alternating order about the same 

 horizontal plane m n o p q r* 



* Two individuals of the same form as that which gave rise to the 

 twin crystal, just described, may penetrate each other in such a manner 

 as to give rise to re-entering angles, as in Fig. 139. But here, the as- 



