48 
NORTH GALLERY. 
[upper 
Div. II. find arseno-sulphides of Cobalt and Iron of the same chemical type 
as Cobalt-glance, but crystallised in the orthorliombic system. Thus 
the three homotypic series of cubic diarsenides, disulphides, and diar- 
senosulphides belonging to the three sections of this division might 
be treated as a single group, while the three corresponding trimetric 
series may be looked on as another such group. 
Sect. iv. Besides the three sections already described, this division contains 
a fourth, wherein metallic sulphides are so combined with sulphides 
Cases 10, of Arsenic, Tin, Iron, &c., as to produce a series of sulphur salts; 
11, 12. in the constitution of which Sulphur plays the part which Oxygen 
plays in the ordinary oxygen-salts. This section is a numerous one 
in point of species, and the following are a few minerals included in 
it that are especially worthy of note. 
In one (and that a somewhat ambiguous) class of these Salts, Iron, 
either as an iron epideutosulphide (Feg S3) or as iron bisulphide 
(Fe 83), would seem to enter as a constituent of the " acid" ingredient. 
Case 10. In this class we meet with two important copper ores, the largely worked 
Chalco-pyrites or Copper-pyrites, and Erubescite or Purple Copper-ore. 
Of both these minerals, there are crystallised specimens from Corn- 
wall ; and massive pieces from Tuscany are seen in the front of Case 7. 
The rare mineral, Sternbergite, consisting of Iron, Sulphur, and 
Silver, belongs also to this class; while Linn^ite, or *' Cobalt-pyrites," 
(C03 S4, cobalt epitrisulphide,) is a sulphur-compound of Cobalt, exactly 
analogous to the oxygen-compounds termed the '* magnetic oxides" 
of Iron or Manganese, 
Oase 11. Tin-pyrites is a bibasic cuprous sulphostannate, containing Iron and 
Zinc, being a salt of the tin disulphide Sn S2, (sulphostannic acid). 
The largest class of the sulphur salts is that consisting of sulph- 
arsenites, sulpho-bismuthites, and sulph-antimonites. Among these 
€ase 11. Tetrahedrite (Fahlore or Grey Copper), is noticeable as a most im- 
portant ore of Copper. It is a sulph-antimonite of that metal, in which 
copper sulphide is frequently replaced by small amounts of silver 
sulphide, and is also associated with sulphides of Iron and Zinc. In 
some of its varieties, as in Tennantite, the Antimony epideutosul- 
phide is entirely, and in others partially, replaced by an equivalent of 
Arsenic sulphide. The argentiferous Tetrahedrite is a valuable ore of 
Silver. Remarkable specimens of Bournonite, a tri-basic sulph-anti- 
monite of Copper and Lead from the Herod's-foot mine in Cornwall, 
Cafie 11. are here in juxtaposition with those from the Hartz, and from Traver- 
sella. Tlie so-called lied Silvers, a group of isomorphous rhombohedral 
minerals, are the tri-basic sulphantiraonite and sulpharsenite of Silver, 
Oaj-;el2. Pynirgyritc and Proustite ; sometimes in a comparatively isolated 
state, liut more frequently blended togethei in various proportions. 
Ikxiutiful as well for their forms as for their blood-red colours, that 
are deeper in tint according as the antimony preponderates over 
nrseiiic, they constitute one of the more precious of the ores of Silver. 
The specimens of Pyrargyrite and Proustite exhibited in Case 12, and 
in particular those of the latter mineral from Chili, are extremely 
fine. 
