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NORTH GALLERY. 
[UPPER 
Div. IV. among them is Cuprite, the red oxide of Copper, cuprous oxide. It 
Case 15. occurs in ruby-coloured and transparent crystals of the cubic system. 
These are seen in the first half of Case 15, and with them are the 
" Tile ore," from the Urals, and the bright-red capillary deposits 
of Chalcotrichite. The cupric oxide, as Melaconite and Tenorite, 
succeeds to the crystalline oxides c f Magnesium (Periclase), and 
of Zinc (red oxide of Zinc, or Ziincite), in the other half of this 
Case. These are followed, first by the hydrated monoxides, including 
Brucite, the magnesium hydrate, 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. The lead-oxychlorides, Matlockite 
and Mendipite, are arranged here with Atacamite, a cupric chlor- 
hydrate, and of Percylite, a beautiful mineral, of which one specimen, 
of uncertain locality, is associated with Gold. It is a hydrated com- 
bination of the oxychlorides of Lead and Copper. 
The next class in the section of oxides is composed of minerals of a 
Case 16. chemical type, similar to that of the magnetic oxide of Iron (the 
("•) ferro-ferric oxide), which may in fact be viewed as a combination of 
ferrous oxide with ferric Oxide, and thus, while possessing the formula 
and a place in the section of the oxides, has claims to be recognised as 
a salt. For the group of cubic-formed minerals to which Magnetite 
more especially belongs, the " Spinel Group," includes Franklinite and 
Chromite (Chromic-iron), which latter mineral is the source of the 
chrome yellow and of some other colouring matters employed in the 
Case 16 (i.) arts. The Spinels, properly so called, also belong to it. These are 
alutninates of Magnesium, of Zinc, Iron, or Manganese; ferric oxide 
occasionally playing the part of alumina. The deep-red " Spinel 
Kuby " and the pale rose-tinted " Balas Ruby " are beautiful gems cut 
from specimens of this Mineral, of which a good assortment of crystals 
is exhibited. Pleonast, Gahnite, Dysluite, are opaque varieties of 
Spinel. To this class also may be referred the Chrysoberyl, a com- 
bination of glucina and alumina (glucinum aluminate), belonging to 
the same type. It is orthorhombic in crystallisation, and as a gem, 
Case 16. known by the name of " oriental chrysolite," it presents itself as a 
<ii.) beautiful greenish-yellow stone, equal in lustre and almost in hard- 
ness to the Sapphire. The variety Cymophane is so named from 
a cloudy appearance that presents itself in two of the faces of the 
crystal, and is retained even when the transparent stone is cut and 
polished. Cut en cabochon, i.e., with a concavo-convex, or with a con- 
vex form, the less transparent specimens furnish one of the kinds of 
stone to which the jewellers give the name of Cat's-eye. Of the dark 
greeu variety from the emerald mines of the Ural, termed Alexan- 
drite, very fine specimens are seen in this Case. It is amethyst- 
coloured by artificial light. 
Case 17. The next class among the oxides is that of the sesquioxides. 
The pure oxide of Aluminium is seen in colourless crystals of 
Corundum, consisting for the most part of hexagonal pyramids 
and prisms. With minute traces of colouring ingredients, these 
It 
