MINERALOGY. Sl 
products. The two latter have the same primary form of crystallization, 
namely, that of figs. 33 and 40, belonging to the trimetric system ; sulphate 
of soda, on the other hand, crystallizes in the monoclinic system; pl. 36, 
jig. 18, represents crystals of sulphate of ammonia; jig. 28, of sulphate. of 
potassa; and fig. 17, of sulphate of soda. 
16. Alum. 
This well known substance, a sulphate of alumina and potassa, is produced 
by the decomposition of alum slate, and forms in minute crystals on the 
surface. By leaching earth containing alum, and evaporating the ley, the 
salt may be obtained in very large masses of crystals, whose primary form 
is the regular octahedron (fig. 24). Octahedral segments (fig. 25) in 
groups, as shown in fig. 16, are, however, more frequently met with. 
17. Borax (borate of soda). 
This substance is used as a flux in melting various metals and solders, 
and is applied to various other technical purposes. It is met with in a 
crude state as tincal, obtained in lumps from certain lakes in Thibet and 
Persia. The refined borax is obtained from tincal by repeated crystallization, 
producing variously modified rhombic prisms. 
18. Saltpetres of potassa and soda. 
The ammonia which forms in the ground by the decomposition of various 
substances containing nitrogen, and diffused in damp air, becomes converted 
into nitric acid by contact with porous solid bodies, as limestone; the acid 
then combines with the bases existing in the soil. Thus the Chilian 
saltpetre is obtained by the saturation of this nitric acid by soda, this nitrate 
of soda crystallizing in obtuse rhombohedrons ( fig. 36), and occurring in vast 
quantities, efflorescing from the surface of the ground, in South America. 
Common saltpetre arises in a similar manner by the conversion of the 
potassa of the soil into a nitrate. This does not occur naturally in large 
masses, but is prepared by artificial processes of decomposition. This 
saltpetre crystallizes in the trimetric system, with primary forms, as in 
jigs. 47 and 48, as also in the modifications (figs. 41 and 43). Its crystals 
are generally hexagonal, with two acute terminations (fig. 26). 
19. Phosphates of alkalies. 
These also are essential to the fertility of a productive soil, although 
occurring always in small quantities. Phosphate of potassa, as obtained by 
evaporation from its aqueous solution, crystallizes dimetrically (fig. 42) : 
phosphate of soda, on the other hand, is monoclinic, and generally in 
modified shapes, the primary forms being represented in pl. 36, figs. 38, 39. 
44, 46, and 49. 
Economica, Uses or Mrinerazs. 
In comparing the different groups of minerals just described, we find a 
great diversity in regard to their economical value. The utility of single 
minerals does not always depend on the properties which render a mineral 
substance fit for a certain end, but rather is based on the degree to which it 
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