MINERALOGY. 63 
Hartz. It forms thin plates, which are rather pliant, and exhibits a cleavage 
into cubes. It is harder than rock salt, is opake, lustrous, iron-black, and 
contains from sixty-five to sixty-seven per cent. of silver. 
2. Selenid of Mercury and Lead. 
This remarkable mineral is exceedingly rare, and is found at Tilkerode 
and Lerbach in the Hartz. It occurs in finely granular, metalloid, bluish- 
grey streaks and spots, in a ferruginous calc spar, as shown by the dark 
portion in pl. 33, fig. 21. A third selenid, that of lead, occurs at Tilkerode, 
under similar circumstances. It is difficult to distinguish from the preceding, 
and furnishes a very argentiferous lead after reduction. It is, however, very 
rare. 
Class 7. Sulphurids, or Sulphurets. 
These are combinations of electro-positive metals with sulphur. They 
all yield the smell of sulphurous acid when heated before the blowpipe on 
charcoal; they are soluble in nitro-hydrochloric acid, with the separation 
of sulphur. 
1. Galena. 
Galena or sulphuret of lead is one of the most important of all minerals, 
furnishing the greater part of the lead of commerce, as also a considerable 
amount of silver. It occurs principally in veins, as in the Hartz, in Saxony, 
in Bohemia, France, England, and Spain. The most abundant deposits 
known are those in the north-western parts of the United States, Missouri, 
lowa, Wisconsin, and Illinois. Galena is a combination of equal atomic 
‘weights of sulphur and lead, or of 104 equivalents of lead and 167 of 
sulphur, with a slight admixture of other sulphurets, as of silver, antimony, 
bismuth, copper, and iron. It most generally occurs in amorphous masses, 
which exhibit crystals in the cavities. The crystals belong to the 
monometric or regular system, the most usual forms being those of 
pl. 33, figs. 35, 33, 41, 43, 44; which, however, like most natural crystals, 
seldom or never occur in so perfect a form. It is more generally the case, 
that apart from incomplete development, the same crystal presents faces 
belonging to several different forms. The cube often appears incomplete 
or distorted, as shown in fig. 20. Other natural crystallizations are shown 
in figs. 11,17, and 19. The crystals of galena, as well as the amorphous 
masses, tarnish on exposure to the air, exhibiting a dark bluish-grey hue ; 
the fresh fracture is leaden-grey, and very lustrous. They have a three-fold 
cleavage, a piece of galena, when broken by a hammer, exhibiting the cubic 
character, as shown in fig. 6. 
The specific gravity of galena is 7.58, its hardness being a little greater 
than that of rock salt. When heated, it breaks up, if struck with a hammer, 
into little cubes; at a high heat it melts and becomes converted into vapor, 
which is decomposed in the open air, with the formation of sulphate of 
lead; if this operation be conducted in closed vessels, the vapor condenses 
in crystals on the sides or colder parts of the furnace. These crystals 
present the appearance of imperfect cubes, as in fig. 8. 
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