84 



MINERALOGY. 



the mixtures of these inter se which are kept secluded from the 

 light. The crystal forms and colour suite of Fluor spar exhibited 

 in Cases 7 and 8, and in the glazed ends of 9 and 10, form a series 

 as remarkable for beauty as any in the Collection. 

 Cases The Salts in the second section are represented by certain double Section I 



9 & 10. fluorides, of which the most important is the Greenland mineral Complex 

 Cryolite (sodium aluminium fluoride), represented by some excellent chloride 

 specimens in its crystallised form. 



Division IY. Compounds of Oxygen. 



The remaining division consists of Minerals of which Oxygen is a 

 constituent ingredient, & class necessarily large on a planet with an atmo- 

 sphere consisting in considerable proportion of this chemically energetic 

 element. The rocks which constitute the earth's crust are aggregates 

 of minerals falling under this chemical division. Here, as in the pre- 

 vious divisions, we distinguish the more simple kinds of combination 

 from the more complex ; and though such a distinction as is expressed 

 by a section of oxides and a section of salts is a difficult one to de- 

 fine with logical precision, it yet serves the object sought in a system 

 of classification, by bringing together compounds that most closely 

 resemble each other, the different classes falling into a natural 

 sequence, nearly in the order of the simplicity of their chemical formulae. 



The first section of this chemical division, the Oxides, will be found Section! 

 arranged in Cases 10 to 15, those containing the greater proportion of Oxides. J 

 oxygen following after those that contain fewer. Commencing with 

 basic types of oxides, we pass through certain comparatively neutral 

 oxides (among which we must look for those members of the section 

 which possess the most equivocal claim to a place in this section); and 

 we then come to the higher oxides which act the part of acids in 

 combining with bases. 



At the beginning of this section are placed the minerals in which 

 oxides or hydrates are combined with chlorides or fluorides, &c. 

 The lead-oxychlorides, Matlockite and Mendipite, are arranged here 

 with Atacamite, a hydrate, combined with cupric chloride, and of 

 Percylite, a beautiful mineral, of which one specimen, of uncertain 

 locality, is associated with Gold. It is a hydrated combination of 

 the oxychlorides of Lead and Copper. 



The oxides include several very important minerals. First in order (a.) Mo 

 among them is Cuprite, the red oxide of Copper, cuprous oxide. It oxides, 

 occurs in ruby-coloured and transparent crystals of the cubic system. 

 These are seen in Case 10a, and with them are the " Tile ore," from 

 Siberia, and the bright-red capillary deposits of Chalcotrichite from 

 the Fowey Consols Mines, Cornwall. The cupric oxide, as Melacohite 

 and Tenorite, succeeds to the crystalline oxides of Magnesium 

 (Periclase), and of Zinc (red oxide of Zinc, or Zincite), in the other 

 half of this Case. These are followed by the hydrated monoxides, 

 including Brucite, the hydrate of Magnesium, which presents delicate 

 hexagonal transparent crystals. 



