450 PHYSIOGRAPHY. CLASS n. 



nical. HAUY. Trait^, T. IV. p. 56. Tabl. comp. p. 95. 

 Traite, 2de d. T. IV. p. 28. 



Fundamental form. Scalene four-sided pyramid. 

 P = 131 51', 105 56', 93 20'. Vol. I. Fig. 9. AP. 



a : b : c = 1 : V 2 ' 83 : V 1 ' 30 - 



Simple forms. P x ; P + oo (M) = III 53'; 



p r _ 1 ( r ) = 145 26'; Pr (s) = 118 32'; 



?r + 1 = 80 8'; Pr + OD; Pr + 1 = 5922'. 

 Char, of Comb. Prismatic. 



Combinations. 1. Pr 1. P -f oo. Fig. 2. Freiberg, 

 Saxony. 



2. Pr. Pr +1. P + OD. Wheal Maudlin, Cornwall. 



3. ?r 1. Pr+1. P-HOO. Ehrenfriedersdorf, 

 Saxony. 



4. P oo. Pr + 1. Pr + 1. P + oo. Wheal 

 Maudlin. 



Cleavage, P -f QD pretty perfect, traces of P oo. 

 Fracture uneven. Surface, Pr 1 deeply 

 streaked parallel to its own edges ; Pr sometimes 

 rough, or striated in the direction of its edges of 

 combination with JPr 1 ; the remaining faces 

 are smooth. 



Lustre metallic. Colour silver. white, inclining and 

 passing into steel-grey. Streak dark greyish- 

 black. 



Brittle. Hardness = 5-5 . . . 6-0. Sp. Gr. = 6-127, 

 of a crystallised variety. 



Compound Varieties. Twin- crystal : 1. Face of 

 composition parallel, axis of revolution perpendicu- 

 lar, to a face of Pr -f 1, the individuals being con- 



