Among the rarer minerals, attention may be called to the fine Div. II. specimens of a variety of Freieslebenite, from Hiendelencina, in Case 12, Spain; also to Fireblende and Xanthocone, the latter containing a sulpharsenate of Silver, combined with the tri-basic sulpharsenite of that metal.

DIVISION III. COMPOUNDS OF THE HALOGEN ELEMENTS.

DIV. III.

Sect. i. This next principal division of the Collection is also subdivided into Cases 13, the simpler compounds, and a more complex section of Salts. Among 14. the former will rank Calomel, Salammoniac, Common Salt (Sodium chloride), and Sylvine, the corresponding potassium chloride, the two latter being crystallised in large cubes and cubo-octahedra. With these are arranged the chloride, iodide and bromide of Silver, and Case 13. the mixtures of these inter se which are kept secluded from the light. 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 Sect. ii. fluorides, of which the most important is the Greenland mineral Case 14. Cryolite. It is the sodium fluo-aluminate, and is represented by some excellent specimens in its crystallised form.

DIVISION IV. COMPOUNDS OF OXYGEN.

Div. IV.

Cases 15, to 60.

The remaining division consists of Minerals of which Oxygen is a constituent ingredient, a class necessarily large on a planet with an atmosphere 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 previous 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 define 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 formulæ. The first section of this chemical division, the Oxides, will be found Section i. arranged in Cases 15 to 26, those containing the greater proportion of equivalents of 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.

The oxides include several very important minerals. First in order among them is Cuprite, the red oxide of Copper, cuprous oxide. It

E

Case 15.

Div. IV. 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 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 preceding 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.

Case 16.

(ij.)

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 ferroso-ferric oxide), which may in fact be viewed as a combination of ferrous oxide with ferric oxide, and thus, while possessing as an epitritoxide the formula and a place in the section of the oxides, has claims to be recognised as a salt.

66

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 arts. The Case 16 (i.) Spinels, properly so called, also belong to it. These are aluminates of Magnesium, of Zinc, Iron, or Manganese; ferric oxide occasionally playing the part of alumina. The deep-red "Spinel Ruby" 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 combination of glucina and alumina (glucinum aluminate), homotypic with the epitritoxides. It is orthorhombic in crystallization, and as a gem, known by the name of "oriental chrysolite," it presents itself as a beautiful greenish yellow stone, almost equal in lustre and in hardness to the Sapphire. 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 cabochon, 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 the emerald mines of the Ural, termed Alexandrite, very fine specimens are seen in this Case. It is amethyst-coloured by candlelight.

Case 16.

(ii.)

Case 17.

The next class among the oxides is that of the epideutoxides (the so-called "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 crystals assume rich hues, and when transparent become

gems conspicuous for their extensive colour-suite, that rank next in Div. IV. value, as in lustre and hardness, to the diamond. These are the colourless Lux Sapphire, the (azure) Sapphire, the Ruby, the OrientalTopaz, Oriental-Amethyst,' Oriental-Emerald,' &c.; gems not to be confounded with those from which they borrow their names, while distinguished from these by their title Oriental," in allusion to the Eastern lands, India, Ceylon, Siam, Pegu, &c., which from the earliest times have produced the gem forms of this mineral in their greatest perfection. In the "Star stones a six-rayed star is seen, of which the position is symmetrical in respect to the morphological axis of the crystal; and through the less pure varieties of Corundum, we descend to the opaque and granular, massive, but still, from their hardness, valuable states of this Mineral, of which Emery is an impure form. Identical in chemical and crystallographic type with Corundum, though Cases 17 very different in aspect both in its crystalline and massive varieties, is the valuable iron ore, Hæmatite, the ferric oxide. A tarnish on some of its crystals, especially on those from Elba, produces an iridescent effect of great beauty. With Hæmatite is placed Ilmenite, or Titanic-iron, one of the ambiguous species of this class. Intimately blended with the former mineral in all proportions and crystallising in its forms, it yet presents the formula of titanate of Iron, a formula, however, which, as containing two equivalents of metal united to three of oxygen, is in fact homotypic with an epideutoxide (sesquioxide).

and 18.

The hydrates of this class include the important iron ore Limonite Cases 18 (Brown-hæmatite), and Goethite, which is monohydrated ferric oxide. (ii.) and 19. In juxtaposition with the fine Cornish specimens of this mineral, from the Restormel mine, are Manganite and Diaspore, respectively the monohydrated manganese and aluminium oxides, isomorphous with Goethite.

The class of dioxides is illustrated by a series of crystals and Case 20. other forms, especially rich in the Cornish varieties-of Cassiterite or Tin-stone (stannic acid), the ore of tin: and in the same Case is placed the Zircon, consisting of the associated zirconic and silicic. dioxides (zirconic and silicic acids). Its crystals, like those of Cassiterite, with which it is nearly isomorphous, are pyramidal. Its pellucid varieties are gems. The dull green is the Jargoon, while peculiar ("hyacinthine") red tints characterize the gem known as the Hyacinth or Jacynth, of which fine cut specimens are in Case 20 (iv.). The yellow and blue tints are rare, but the more pellucid and colourless zircon, from its exceptionally high refractive power, approaches even the diamond in brilliancy.

In the same continuous series is Rutile, the titanium dioxide Case 21 (titanic acid), isomorphous with Zircon, and approximately so with (i.) Cassiterite. Anatase is the same substance, also in pyramidal forms, but with different parameters; while in yet a third series of forms this trimorphous titanic dioxide is to be seen as the orthorhombic mineral Brookite, of which the specimens from the Snowdon district are remarkable.

In this Case is also exhibited the manganese dioxide (the “per

Div. IV. oxide"), Pyrolusite, the mineral employed for the production of oxygen gas, and for the evolution from the chlorides of their chlo rine, so largely employed in the arts.

Cases 21

The Rhombohedral system is represented in the class of the (ii.) to 25. dioxides by Quartz and its varieties.

Cases 21 to

29.

Cases 25

Case 21

(ii.)

This important mineral is silica, the silicon dioxide (silicon being an 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 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 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 distinguished by their optical properties.

Case 22.

25.

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 presenting the forms of Datholite (compare Case 51).

35.

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 several salts of the metals Calcium, Barium, Strontium, Lead, and Magnesium, and of the corresponding irou and manganese salts with them, finds 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, Strontianite the strontium carbonate, and Cerussite the lead carbonate. The specimens of this last mineral and those of Witherite are especially noticeable.

The second series comprises those minerals of this chemical type Div. IV. that crystallise in rhombohedral forms isomorphous with those of Calcite, the rhombohedral calcium carbonate. These include the magnesium carbonate, Magnesite; zinc carbonate, Calamine; and the iron and manganese salts termed Chalybite and Rhodochroisite respectively. They include also the mixtures of these in a very considerable variety, such as Dolomite, Ankerite, Brown Spar, &c. Baryto-calcite crystallises in forms of the clinorhombic system, and establishes the trimorphism of these minerals by exhibiting the barium and calcium carbonates crystallised in a third set of distinct crystalline forms. The crystals of Calcite in Cases 29 to 31, and in the fronts of Cases 27 to 29, are a very fine series, as well for their varied forms as for the conspicuous illustrations certain of them afford of the highly double-refracting property of the crystal. Some singular pseudomorphs from Devon, in the Chalybite Case, are well worthy of notice.

The Limestone and Dolomite rocks are formed of Minerals from this series, in various massive, granular, or crystalline aggregations, the latter of which frequently förm Marbles; while into the Clay-ironstone, with which the blast furnaces of Wales and Scotland. have been largely fed, spathose-iron, or Chalybite, enters as an ingredient in a high percentage.

Among the hydrated carbonates, and carbonates combined with Case 34 (ii.) hydrates, or with compounds belonging to the previous divisions, attention may be called to the green and blue copper ores, Malachite and Chessylite, of which latter a very fine series of crystals is exhibited.

Case 35 contains also fine specimens of Phosgenite, a combination Case 35 (ii.) of the chloride and carbonate of lead; and of Parisite, an analogous compound from the Emerald Mines of Santa Fè di Bogotà, containing the fluoride combined with the carbonate of calcium and of the rare metals of the cerium group.

The Silicates, occupying no less than fifteen Cases, form the next class in this section. The minerals comprised in this large, varied, and important class are arranged in series distinguished by the type of oxide that characterises the bases in the silicate. Thus the silicates corresponding to monoxide-bases (ferrous oxide, magnesia, &c.) are arrayed in one series; those the bases of which are epideutoxides (sesqui-oxidès) are in another; and such as contain bases of both kinds fall into a third The respective hydrates are comprised under the series to which the minerals of corresponding anhydrous types belong.

The first of these series is composed of such silicates as are formed Case 36 (i. by silicic acid in association with monoxides only, or in which epideutoxides ("sesqui-oxides") are met with only as accidental or intrusive ingredients. The anhydrous section of this series contains, among others, the following minerals. Phenakite, the di-glucinum silicate, and Wille

zinc-silicate corresponding and isomorphous with it, represent a rhombohedral series of dibasic silicates. The specimens of Phenakite from the emerald mines of the Urals are extremely fine. Of the same chemical type are the minerals comprised in the Olivine group, which