of the Trimetric System. 



TABLE III. 



Angle of Vertical Prism, near 109° 28'. 



59 



I. 



Marcasite, . . . 



II. 



Mispickel, . . . 



Leucopyrite, . . 



Aurotellurite, . . 



Dome 

 li. 



Axes 



b 



1.5737 : 1 : 1.3287 

 1,7588 : 1 : 1.4793 

 1.7723 : 1 : 1.4496 



The angle of the vertical prism in Table III. is near the 

 angle of a regular octahedron (109° 28'). As this prism is a 

 cleavage prism, and the only distinct one in the species, it ap- 

 pears to be the true vertical prism. 



But if we give the species another position, we may exhibit 

 a relation to sections II. and III. of Table II. ; and as they are 

 all related to the species of those sections in composition, this 

 relation is of fundamental interest. Making the brachydome 11 

 the vertical prism, then the angle given above for the vertical 

 prism is the new macrodome, and the supplement of that for 

 the macrodome is the new brachydome. This gives for Mis- 

 pickel the angles /: /= 99° 52'; li : li ■= 111° 53' ; It : 11 = 

 120° 46'. If we now double the length of the vertical axis, the 

 dome 111 53' becomes §t, and 120° 46' ii; and the three angles 

 will be 



1:1= 99° 52' 5 H : U = 72° 58' ; It : It = 82° 40', 



which are almost identical with the angles in Orpiment. The 

 following table presents the angles and axes of the species thus 

 changed in position, and also those referred to on Table II. 



It appears from the table that Marcasite, Fe S 2 , is very near Sul- 

 phur in its angles and axes ; while A urotellurite (Ag, Au), Te 2 , and 

 Mispickel, Fe (S, As) 2 , to which Leucopyrite, Fe As 4 , should be 

 added, have the form nearly of Orpiment. It is a question, 

 therefore, whether Table III. should not be suppressed, and the 



