of Mineral Species of the Trimetric System. A9 
Other species in Table II require special remarks. 
In Sulphur, the unit macrodome of authors has the summit angle 
46° 18’. But taking 32 as the unit dome, the angle,is near that of Ba- 
rytes and the other sulphates, as in the table; 42 in sulphur is near 42 in 
Barytes. The homeomorphism of sulphur and the sulphates (RO, SO°) 
is hence evident. 
Orpiment (sulphuret of arsenic) differs from sulphur in pertaining to 
Section If. The sulphur and arsenic compounds present a like amount 
of difference; and, further, they show that the fundamental vertical 
prism is that of 100° 40’, instead of that of 117° 49’, adopted by some 
authors. The difference in the unit domes of sulphur and orpiment is 
about 74 degrees, and the difference in the sulphurets and arseniurets 

sures. We here make Meee, 
B of Scacchi the terminal plane O; A, the plane i; C, the plane 
#3; also 92 j and m is 1 or the unit octahedron. In Scacchi’s 
is $7, 
type II (figure d above), the planes Joost same fundamental 
“pl. 4), 
pl to the A 
form, are 3% (e of Scacchi, fig. 13, if (i), $3 (m), 3 (07). 
* Memorie Geolgiche sulla Campania per A. Scacchi, Napoli, 1849, p. 120. 
Secon Series, Vol. XVIII, No. 52.—July, 1854. 1 
