FLOOR.] 
MINERALS. 
45 
chloride, iodide and bromide of Silver, with the mixtures of these, Case 13. 
inter se. These last are kept secluded from the light, but the crystal 
forms and colour suite of Fluor spar exhibited in Case 14, form a series 
as remarkable for beauty as any in the Collection. The Salts in this 
division are represented by certain double fluorides, of which the most 
important is the Greenland mineral Cryolite. It is a fluo-aluminate Case 14. 
of Sodium. 
Div. IV. Compounds of Oxygen. 
The remaining division consists of Minerals of which Oxygen is a Cases 1/ 
constituent ingredient, a class necessarily large on a planet with an atmo- GO. 
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- 
tine 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 formula?. 
The first section of this chemical division, the Oxides, will be found Section 
arranged in Cases 15 to 26, those containing the greater proportion of 
equivalents of oxygen following after those that contain fewer. Com- 
mencing with basic types of oxides, we pass through those of compara- 
tively 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 from these we attain to the higher oxides which act the 
part of acids. The oxides include several very important minerals. 
First in order among them is Cuprite, the red oxide of Copper, cuprous 
oxide. It occurs in ruby-coloured and transparent crystals of the cubic 
system. These are seen in the first half of Case 15, and with them Case 15 
are the " Tile ore," from Siberia, and the bright-red fur-like deposits 
of Chalcotrichite. The cupric oxide, as Melaconite and Tenorite, 
succeeds to the crystalline oxides of Magnesium (Periclase), and 
of Zinc (red oxide of Zinc, or Spartalite), in the other half of this 
Case. These are followed, first by the hydrated monoxides, including 
Brucite, the hydrate of Magnesium, which presents delicate hexagonal 
transparent crystals; in succession to which are minerals in which 
oxides of this type are associated with compounds belonging to preced- 
ing Divisions of the Collection. With the rare and almost exclusively 
British oxychlorides of Lead, Matlockite and Mendipite, and with Ata- 
camite, a hydrate, combined with chloride of Copper, is also arranged 
the unique specimen of Percylite, a beautiful mineral of unknown 
locality, associated with Gold. It is a hydrated combination of the 
oxychlorides of Lead and Copper. 
The next class in the section of oxides is composed of minerals of a 
chemical type, similar to that of the magnetic oxide of Iron (the 
