DESCRIPTION AND ANALYSIS OF YENITi:* 



141 



Geometrical characters *. 



Cleaving exhibits indications of laminee parallel to the Mechanical 

 sides of a prism with a rhombic base, the angles of which divisioa. 

 are 112° 37' 9 "and 67° 22' 51". Rather more evident in- 

 dications are found of a division according to the shorter 

 diagonals of the rhombs. This section is pointed out on 

 the crystals by the striae at the summit. The bases present 

 no section : their fracture on the contrary is uneven conchoi- 

 dal, &c. 



The primitive form, PI. IV, fig. 3, is a right prism, with a PrimitLveform. 

 rhomboidal base, the diagonals of which are to each other as 

 2 to 3. From the theory of decrements its height is to the 

 shorter diagonal as 4 to v'7. 



The crystals have five varieties with respect to figure. Figures of the 



Var. 1. Fig. 4 is the primitive form elongated, and termi- crysuU. 

 nated by a pyramid with four faces rising from its edges. 

 The angle of incidence between M and O is 128° 28' 59"; 

 that between O and O, 139° 36' 48"; and that between O 

 and its reverse 1 1 7° 38' 8'". 



Var. 2. Fig. 5 is a tetraedral prism, nearly rectangular, 

 terminated by a double bevil, obtuse, and placed on the 

 obtuse angles. The angle of incidence between S and S 

 is 83° 16' 4"; that between R and its opposite face llS" 

 2" 9". 



Var. 3. Fig. 6 is the preceding form with a double trun- 

 cature at each acute angle of the bevil. The angle of inci- 

 dence between O and R is 159° 48' 24". 



Var. 4. i^ig. 7 is an octaedral prism terminated by an 

 obtuse octaedral summit, four of the faces of which are at 

 the angles of the prism, and four on the edges. The angle 

 of incidence between X and the edge Z is 131° 24' 37". 



Var. 5. Fig. 8 exhibits the preceding variety with this dif- 

 ference, that it has at the summit a facet parallel to the base 

 of the primitive form. The angle of incidence between P 

 and R is 146° 31' 43"; and that between P and O, 141° 

 31' 1". 



* These characters were ascertaiaed by Mr. Cordiar, engineer of 

 mines, who was so obliging a;j to undeiutke the examination of the 

 crystalline forms, which he c^lcukt:;d accoiding to the method of our 

 learned comrade Haiiv, 



At 



