50 
NORTH GALLERY. 
[upper 
Div. IV. occurs in ruby-coloured and transparent crystals of the cubic system. 
Ca£e 15. 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 of Magnesium (Periclase), and 
of Zinc (red oxide of Zinc, or Zincite), 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 hydrate, combined 
with cupric chloride, and of Percylite, a beautiful mineral of uncertain 
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 
Case 16. ferroso-ferric oxide), which may in fact be viewed as a combination of 
("•) ferrous oxide with ferric oxide, and thus, while possessing as an epi- 
tritoxide the formula and a place in the section of the oxides, has 
claims to be recognised as a salt. 
The group of cubic-formed minerals to which Magnetite more espe- 
cially belongs, the " Spinel Group," includes Franklinite and Chro- 
mite (Chromic-iron), which latter mineral is the source of the chrome 
yellow and of some other colouring matters employed in the arts. The 
Case 16 (i.) Spinels, properly so called, also belong to it. These are aluminates 
of Magnesium, qf Zinc, Iron, or Manganese ; ferric oxide occasionally 
playing the part of alumina. The deep-red " Spinel Kuby " and the 
pale rose-tinted " Balas Kuby " 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 combination of glucina 
and alumina (glucinum aluminate), homotypic with the epitritoxides. 
It is orthorhombic in crystallization, and as a gem, known by the 
Case 16. name of " oriental chrysolite," it presents itself as a beautiful greenish 
(ii.) yellow stone, almost equal in lustre and in hardness to the Sajjphire. 
The variety Cymophane is so named from a cloudy appearance that 
presents itself in two of the planes of the crystal, and is retained even 
when the transparent stone is cut and polished. Cut en caboclwn, 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 green variety 
from tl)o emerald mines of the Ural, termed Alexandrite, very fine 
specimens are seen in this Case. It is amethyst-coloured by candle- 
light. 
Cu'^c 17. The next class among the oxides is that of the epideutoxides (the 
eo-callcd " sesquioxides"). The pure oxide of Aluminium is seen in 
colourless crystals of Corundum, consisting for themost part of hexa- 
gojial pyramids and prisms. With minute traces of colouring ingre- 
dients, these crystals assume rich hues, and when transparent become 
