62 MINERALOGY. 
per cent. of cobalt, and seventy-four of arsenic. Metallic cobalt, principally 
obtained from this arseniate of cobalt, is a refractory metal, similar to iron, 
and like it magnetic. The combinations of its oxydes form very valuable 
coloring matters, particularly for blue. To prepare these the ore must be 
freed from the arsenic, forming its larger portion, by roasting it in the air, 
the result of which is to drive off the arsenic in white fumes of arsenious 
acid. The resulting ore may be applied to many purposes. Thus it 
imparts a deep and beautiful blue to glass, for which it is much used. 
Melted with quartz it forms zaffre, safflor, or zaftra, used in the preparation 
of smalts. This is a glass formed from quartz and potash, colored by zafire. 
The cobalt colors, known as Leitner’s Blue and Thenard’s Blue, are of 
great beauty. The first is obtained by calcining alumina with nitrate of 
cobalt; the latter by calcining alumina with phosphate of cobalt. Both 
remain unchanged in fire, for which reason they are applicable in painting 
porcelain and glass. 
4. Glance Cobalt. 
This mineral, like the last, is very rich in cobalt, for which reason it is 
much used in smalts works. It occurs in beautiful crystals of the regular 
system. The most common forms are shown in pl. 33, figs. 57, 61, 62, 50, 
64, 60, 54, 55, 49, 58. It possesses a metallic lustre, is opake, with the 
fresh fracture of a silvery-white color, inclining to red, which soon 
overspreads the surface, and presents a beautiful play of metallic colors. It 
is brittle, and the fracture is uneven, somewhat conchoidal ; when heated it 
gives off arsenious and sulphurous acid. It consists of 33.10 per cent. of 
cobalt, 43.46 of arsenic, 20.08 of sulphur, and 3.23 of iron. It occurs in 
veins and beds of primitive rocks in Norway, Sweden, and Alsace. 
Class 6. Selenids. 
The minerals of this class are combinations of selenium with electro- 
positive metals. They are readily recognised by the peculiar odor of 
burning selenium, when the substance to be tested is heated on charcoal 
before the blowpipe. A smell is developed, if selenium be present, similar 
to that of decaying horse-radish. 
Selenium, when solid, is of a dark-brownish red color on the surface, the 
conchoidal fracture exhibiting a metallic leaden-grey tint. When in the 
finely divided state, obtained when reduced from the watery solution of 
selenious acid, it is of a vermilion red color. Thin splinters of compact 
selenium are somewhat transparent. It burns like sulphur, when heated, 
diffusing its characteristic odor, sensible even in very small quantity. In 
its general peculiarities and combinations it has much resemblance to 
sulphur. In and of itself it is of no practical importance ; its combinations 
are some of them valuable for the metals they contain. The principal of 
these combinations are: 
1. Selenid of Silver. 
This mineral occurs in Mexico, and more rarely at Tilkerode in the 
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