80 MINERALOGY. | 
oxyde. This substance is also known under the name of copperas.  »pper 
or blue vitriol is also well known, this crystallizing in the form of figs. 34 
and 35, pl. 36, and in crusts as in fig. 19. It is of a beautiful blue color, 
and, with the preceding, is much used in dyeing: white vitriol, the sulphate 
of zinc, is also used in the arts. “It occurs in very decomposable prisms 
( fig. 23) of the trimetric system. The crystals are sometimes as in fig. 42. 
9. Fluor-spar (fluoride of calcium). 
This beautiful mineral is extensively distributed, and may be used for 
various purposes of ornament. From it is obtained fluoric acid, which may 
be used for etching glass. A druse of fluor is represented in pl. 36, fig. 32. 
10. Wolfram (tungstate of iron and manganese). 
This mineral is found in the gneiss of Bohemia and Saxony, in the form 
of large black striated crystals, with a lustrous surface ( pl. 36, fig. 15). 
11. Yellow lead ore (molybdate of lead). “So 
This, associated with ores of lead and zinc, occurs, in beds and veins, in 
various parts of Germany, Hungary, Mexico, and North America. It 
crystallizes in variously modified square octahedrons, and occurs, as in 
Jig. 22, finely crystallized, and in dirty yellow masses. 
12. Pyromorphate, or green lead ore. 
This is a phosphate of lead containing some chloride of lead. It is found 
in Bohemia and Saxony, is of a pistachio-green color, and sometimes occurs 
in small green crystals of the hexagonal system, strewed as it were over the 
surfaces of other minerals. fig. 10 represents a specimen from Zellerfeld. 
13. Wavellite. 
This substance (shown in pl. 36, fig. 5) occurs in concentric radiated 
crystallized groups. It is a phosphate of alumina, and not very abundant. 
14. Rock salt. 
Rock salt constitutes vast subterranean beds, which yield their contents 
either to actual mining operations, or by solution in salt springs. This salt 
or chloride of sodium crystallizes in cubes, which are generally distorted 
into rectangular parallelopipedons (fig. 30). In itself it is colorless, but 
often variously tinged by bitumen. Salt springs may be made to yield 
their solid contents by the evaporation of their waters. This evaporation 
may be brought about by boiling, or by exposure to the sun in shallow pans. 
In some places the stream of salt water is permitted to run for a long 
distance through a dense thorn hedge. In this way a considerable evapora- 
tion is effected by the extended surface exposed to the air. The water is 
also obtained in a purer form, as the other constituents of the spring, 
especially the gypsum, dissolved in all such waters, are, to a great extent, 
separated by the evaporation, and the thorns coated so as to present the 
appearance seen in pl. 36, fig. 11. The crystals in which the salt is 
separated from its solution by boiling are mostly incomplete cubes, generally 
with more or less perfect square funnel-shaped cavities. 
15. Sulphates of the alkalies. 
The three most common alkalies, or their combinations with sulphur, 
form very important constituents of the soil. These sulphates are those of 
soda, of potassa, and of ammonia, all of which are well known as artificial 
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