MINERALOGY. 67 
hemihedral crystallizations of the regular system. Thus there are 
tetrahedrons variously modified by faces of other forms of the same system. 
The most common forms of these crystals are seen in figs. 51, 52, 53, 56, 
59. The tetrahedrons are often coated with a thin layer of copper pyrites, 
and then exhibit a beautiful golden lustre, these occurring at times of large 
size. The argentiferous grey copper ores are very valuable silver ores ; 
that known as the graugiltigerz contains 13-18 per cent., and the 
weissgiltigerz nearly 31 per cent., of silver. The proportion of copper in 
this ore sometimes amounts to 30-40 per cent.; in the hydrargyrous ores 
the per-centage of copper is 34-36, and of mercury 2-8. 
8. Cinnabar. 
This splendid red mineral is a sulphuret of mercury, and is valuable as 
being the chief ore of mercury. It occurs crystallized, as in pl. 33, fig. 
25, combined with carbon and alumina, as hepatic cinnabar or liver ore, or 
lamellar as in the coral ore. Its geological position is in grauwacke, and in 
transition sandstone and limestone; being found generally in beds, more 
rarely in veins. It is generally accompanied by calc-spar, native mercury, 
and amalgam. The mines of Almaden and Idria furnish the greatest 
quantity of cimnabar. Large mines have recently been opened in California. 
To obtain metallic mercury, this ore is heated in iron vessels, with iron or 
lime, by which the mercury is separated from the sulphur. Another 
process is to heat the cinnabar in furnaces so constructed that all the 
vapors generated are carried through long galleries, into chambers where 
they are condensed. Mercury and its ores are not generally distributed, 
and for this reason the metal bears a high price. It is used as a medicine, 
and also for coatings of glass mirrors: for many philosophical instruments, 
and for chemical investigations it is indispensable. 
9. Realgar and Orpiment. 
Both of these substances have already been mentioned under the head of 
arsenic as artificial products. The natural realgar occurs in earthy and 
amorphous masses, or in rhombic prisms (fig. 39). . It is a bisulphuret.of 
arsenic, and possesses an aurora-red color, with tolerable transparency. 
Native orpiment is a tersulphuret of arsenic, is of a fiery yellow color, and 
is used in painting and dyeing, as also as an addition to lead in the manu- 
facture of shot. | 
Class 8. Oxydes. 
Combinations of oxygen with electro-positive metals. 
1. Oxyde of Iron. 
Of all minerals embraced in the class of oxydes, that of iron is unquestion- 
ably the most. important. It is diffused over the whole earth, and is the 
principal material from which metallic iron is procured. It occurs in various. 
forms, and is a constituent in all rocks, and in most minerals. Oxyde of iron, 
in and for itself, and in combination with water, forms several distinct 
minerals. Thus, crystallized peroxyde of iron forms specular iron. This 
occurs in very perfect crystals of metallic lustre, and occasionally splendent. 
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