of the Trimetric System. 



57 



sition, falls into the same group with Orpiment, and is near it in 

 angle. Professor Scacchi, in describing Dimorphine,* recog- 

 nises the fact that it affords two angles approaching those of 

 Orpiment, viz. 83° 40' and 117° 48' ; and he adds correctly, that 

 they do not, however, correspond in position in the two species. 

 But on examining further his type I., and viewing the form in 

 a different position, we find that there are two prisms, which taken 

 as domes give the an- 

 gles at summit 83° 

 40' and 75° 40' (an- 

 gles o : o and e : e in 

 Scacchi, pi. 12, f. 4, 

 or 12 and ll in the 

 annexed figure 1) ; 

 and these angles are 

 so near two domes in Orpiment that we can hardly hesitate as 

 to regarding this the right position for the figures. We here 

 make B of Scacchi the terminal plane ; A, the plane il ; C, 

 the plane il ; also o" is ii, and m is 1, or the unit octahedron 

 In Scacchi's type II. (figure 2, above), the planes referred to 

 the same fundamental form, are f* (e of Scacchi, fig. 13, pi. 4), 



*? C0> f i ( w )> ^* (° 2 )- I n tn ^ s tv P e > tne an g^ es j as given in the 

 table, are almost identical with those of Orpiment. The axes 

 become for 



5=1 a=l 



Type I, a : b : c = 1.2876 : 1 : 1.1526 = 1 : 0.77661 : 0.89526. 

 Type II, a : b : c = 1.3262 : 1 : 1.2030 = 1 : 0.75405 : 0.90707. 



The ratio £ in Type II. loses its improbability, if any there 

 be, when it is observed that the domes of this ratio have approxi- 

 mately the angles of the unit domes of sulphur or of the section to 

 which sulphur belongs, they being |2 = 62° 12' (e : e, f. 13, of 



Memorie Geologiche sulla Campania per A. Scacchi, Napoli, 1849, p. 120 



