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NORTH GALLERY. 
[UPPER 
Drv. II. nn( ^ arsenio-sulphides of Iron and of Cobalt with Iron of the same 
chemical type as Cobalt-glance, but crystallised in the orthorhombic 
system. Thus the three homotypic series of cubic diarsenides, di- 
sulphides, and diarseniosulphides 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 sesquisulphide (Fe 2 S 3 ) or an iron persulphide 
(Fe S 0 ), 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 Linnaeite, or " Cobalt-pyrites," 
(Co 2 S s , CoS,) is a sulphur-compound of Cobalt, exactly analogous to 
the oxygen-compounds termed the " magnetic oxides" of Iron or 
Manganese. 
Case 11. Tin-pyrites is a dibasic cuprous sulphostannate, containing Iron and 
Zinc. 
The largest class of the sulphur salts is that consisting of sulph- 
arsenites, sulpho-bismuthites, and sulph-antimonites. Among these 
Case 11. Tetrahedrite (Fahlerz or Grey Copper ore) is noticeable as a most 
important ore of Copper. It is a tetra-basic sulph-antimonite of that 
metal, in which the copper is frequently replaced by small quantities 
of silver, and is also associated with sulphides of Iron and Zinc. In 
some of its varieties, as in Tennantite, the Antimony trisulphide 
is entirely, and in others partially, replaced by an equivalent of Arsenic 
trisulphide. 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, 
Case 11. are nere in juxtaposition with those from the Hartz, and from Traver- 
sella. The so-called Red Silvers, a group of isomorphous rhombohedral 
minerals, are the tri-basic sulphantimonite and sulpharsenite of Silver, 
Case 12. Pyrargyrite and Proustite ; sometimes in a comparatively isolated 
state, but more frequently blended together in various proportions. 
Beautiful as well for their forms as for their blood-red colours, that 
are deeper in tint according as the antimony preponderates over 
arsenic, 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. 
