194 
NORTH GALLERY 
[upper 
Div. IV. oxygen gas, and for the evolution from the chlorides of their chlo- 
rine, so largely employed in the arts. 
Cases 21 The Khombohedral system is represented in the class of the 
(ii.) to 25. diodes by Quartz and its varieties. 
Cases 21 to This important mineral is silica, the oxide of silicon (silicon being 
Cases 25 &n element of the carbon group). This oxide occurs in a state 
(ii.) and 26 physically distinct from Quartz, in the Opal, which is amorphous: 
(i.) specimens of it will be found at the end of the crystalline series of 
the dioxides. Among the purer varieties of these are the Mexican Fire 
Opal, and the beautiful and almost exclusively Hungarian gem, the 
Noble Opal, conspicuous for its fascinating play of colours. Specimens 
Case 21 of Tridymite will be seen in Case 21 ; it is a crystallised form of silicic 
acid, with the specific gravity of opal. Its crystalline forms, however, 
are distinct from those of Quartz, which is the more common and 
Case 22. more dense variety of Silica. The latter is seen in its purest form as 
Quartz crystal in Case 21. Its tinted specimens may vie in point of 
colour with jewels of denser substance and higher refrangibility. 
Among these are the lilac-hued specimens of the Amethyst, the 
Brazilian specimens of which, as well as of the yellow kind, show 
the "rippled" fracture which distinguishes them from the ordinary 
Quartz, with its smooth conchoidal fracture. They are further dis- 
tinguished by their optical properties. 
Cases 23 to A series of minerals succeeds, formed by mixtures of the crystalline 
with the opaline silica, and of these with iron oxides and argillaceous 
and other impurities. They include the various kinds of Jasper and of 
Chalcedony, Prase, Bloodstone and Heliotrope, Hornstone, Carnelian, 
Sard, Plasma, while the various banded, ribbed, eyed, spotted, clouded, 
and other fantastically figured and coloured stones of the Agate kind, 
including Onyx and Sardonyx, in every gradation of translucency, 
illustrate the modes in which these mixed minerals occur, and often 
evidence the successive action of the processes that formed them. Of 
the pseudomorphism of minerals, a good example is furnished in 
Case 24 (i.) Haytorite, a mineral composed of a chalcedonic Hornstone, but pre- 
senting the forms of Datholite (compare Case 51). 
Section ii. We next enter on the section of Oxygen Salts, the first class under 
Cases 27 to which is occupied by the Carbonates. The isomorphous character of the 
35. _ several salts of the metals Calcium, Barium, Strontium, Lead, and Mag- 
nesium, and of the corresponding iron and manganese salts with them, 
fi nds illustrations in the long array of the anhydrous carbonates which 
are here exhibited, crystallised severally in forms which are equivalent 
or united in various proportions of admixture in the same crystal. 
These carbonates are divided by their crystalline forms into two 
large series or groups. The first comprises those crystallising in 
forms on the type of Aragonite, the orthorhombic calcium carbonate. 
Among these are, besides Aragonite, Witherite the barium carbonate, 
mtianite the strontium carbonate, and Cerussite the lead carbonate. 
The specimens of this last mineral and those of Witherite are espe- 
cially noticeable. 
The second series comprises those minerals of this chemical type 
