Imagens das páginas
PDF
[ocr errors]

Properties.—The crystals of this system have the following properties: they are equiaxed singly refracting, equiexpanding and equielastic. We assume their molecules to be spherical.

When examined in the polariscope they present no traces of colour.

Exceptions.—A few exceptions exist to some of the preceding statements; but they are probably more apparent than real.

1. Several crystals of this system, as the diamond, fluor-spar, alum, and common salt, sometimes exhibit traces of a doubly refracting structure. But this is ascribable to irregularities of crystallization, or to the operation of compressing or dilating forces.

2. Boracite (a compound of boracic acid and magnesia) crystallizes in the general form of the cube; the edges of which are replaced, and the diagonally opposed solid angles dissimilarly modified. Instead, however, of being merely a single refractor, as its shape would lead us to expect, Dr. Brewster found that it was a double refractor, with one positive axis of double refraction in the direction of a line joining two opposite solid angles of the cube. So that, in point of fact, it possesses the properties of a rhombohedric crystal. We may, therefore, regard it as a rhombohedron, whose angles differ from a right angle by an infinitely small quantity.

3. Analcime or cubizite (hydrated silicate of alumina and soda) constitutes another remarkable exception to the general rule, that crystals of the cubic system are devoid of a doubly refracting structure. The most usual form of this crystal is the icositetrahedron. Now if we suppose, says Dr. Brewster, its contained cube " to be dissected by planes passing through all the twelve diagonals of its six faces, each of these planes will be found to be a plane of no double refraction or polarization." All intermediate portions doubly refract. From every other known doubly refracting crystal, analcime differs in the circumstance, that all its particles do not equally possess the property of double refraction, those in the planes above mentioned being devoid of this power, and the others possessing it in proportion to the squares of their distances from these planes. It differs from unannealed glass in the fact that a change in its external form does not give rise to a change in its polarizing power; but each fragment possesses the same optical property, when it is detached from the mass, that it had when naturally connected with its adjacent parts. Analcime, therefore, is a complete optical anomaly.

It has been suggested, that these curious optical properties may depend on the presence of both a doubly and a singly refracting mineral; and the fact, that the large opake crystals of analcime, found in the valley of Fassa in the Tyrol, are traversed by plates of apophyllite (a doubly refracting crystal), lends suport to this hypothesis.

[graphic]

SYSTEM H.

THE SQUARE PRISMATIC SYSTEM.

Synonymes.—The four-membered or two- and one-axed, the pyramidal, the tetragonal, or the monodimetric system.

Forms.—The forms of this system are either homohedral or whole forms, or hemihedral or half forms.

[merged small][merged small][graphic]

Four forms of the Square Prismatic System; viz., Two Square Pi isms in different

positions, and Two Oclohedra with square bases.

a a. Principal axis. bb.ee. Secondary axes.

Crystals.—Among the crystals* of this system are chloride of mercury (calomel), bicyanide of mercury, ferrocyanide of potassium (yellow prussiate of potash), peroxide of tin, copper pyrites, zircon, and apophyllite.

Properties.—The crystals of this system have the following properties: They are di-unequiaxed, doubly refracting with one optic axis, di-unequiexpanding, and di-unequielastic. We assume their molecules to be either prolate or oblate spheroids.

The two equal rectangular geometric axes of this system are called secondary axes ; while the third or odd one, which may be greater or less than the others, is the principal or prismatic axis, or the crystallographical axis, or the axis of symmetry. The optical characters of this system are the following: The crystals are doubly refracting, with one optic axis which coincides with the principal axis.

If a thin slice of a crystal of this system, cut perpendicularly

* Sowerby (Ann. Phil xvi. 223.) mentions crystals of Palladium in the form of octohedra with a square base and of symmetrical prisms.

F 2

to the principal axis, be placed in the polariscope, it presents a system of circular rings, with a cross, which is either black or white, according to the relative positions of the polarizer and analyzer.

Ferrocyanide of potassium (commonly called prussiate of potash) may be conveniently used to show these effects. As found in commerce it usually occurs in the form of truncated octohedrons having a square base. It should be split with a lancet in the direction of its laminse, that is, perpendicularly to its principal axis. Plates, of about a quarter of an inch or more in thickness, serve for the polariscope. They present a cross, and a negative system of circular rings; but the yellow colour of the crystal affects the brilliancy of the tints.

Zircon (a compound of silica and zirconia) is valuable for optical purposes, on account of its being a positive uniaxial crystal. Hence if a plate of it, which gives a system of rings of the very same size as that produced by a plate of Iceland spar (a negative uniaxial crystal) be superposed over the latter plate, the one system of rings is completely obliterated by the other; and the combined system exhibits neither double refraction nor polarization.

I shall defer all explanation respecting the rings and cross of this system, until I speak of Iceland spar (a crystal of the rhombohedric system).

Exceptions.—Some exceptions to the above mentioned properties of the crystals of this system exist, and require to be noticed.

1. Ferrocyanide of potassium is subject to irregularities of crystallization; and certain specimens present a double system of rings, or, in other words, are biaxial. Certain uniaxial specimens give a positive system of rings.

2. Apophyllite or Fisheye-stone (a compound of silica, lime, potash, and water) possesses some remarkable properties. In the most common variety, that from Cipit in the Tyrol, the diameters of the rings are nearly alike for all colours — those of the green rings being a little less. Some specimens of apophyllite, called by Dr. Brewster tesselated apophyllite, present, in the polariscope, a tesselated or composite structure, instead of the ordinary cross and circular rings. They will be described hereafter among 'he tesselated or intersected crystals.

SYSTEM m.

THE RHOMBOHEDRIC SYSTEM.

Synonymes.—The three- and one-axed, the klinohedric, the hexagonal, or the trimetric system.

Forms.—The forms of this system are either homohedral or hemihedral.

[graphic]
[merged small][merged small][graphic]

Three forms of the Rhombohedric System; viz., the Hexagonal Prism, the Scalene

Dodecahedron and the Rliombohedron.

a a. The principal axis. bb,cc,dd. The secondary axes.

Crystals.—To this system belong- some bodies supposed to be simple or elementary; viz., antimony, arsenicum, and tellurium.*

Plumbago or graphite and the native alloy of iridium and osmium also belong to this system.

Ice, magnetic iron pyrites, cinnabar, chloride of calcium, Iceland spar, carbonates of iron and zinc, dolomite (magnesian carbonate of lime), nitrate of soda, hydrate of magnesia, tourmaline, talc,beryl, chabasite, quartz, and one-axed mica belong to this system. And here it may be necessary to remark, that the substance known to mineralogists by the name of mica, and which, in trade, is usually but improperly termed talcf, varies in its crystalline forms and optical properties. One kind crystallizes in regular hexagonal prisms, which cleave with extreme facility in one direction, viz., perpendicularly to their axis. This has only one axis of [no] double refraction, and consequently when a lamina of it is placed in the polariscope it presents only one system of circular rings traversed by a cross. This is the kind called rhombohedral or uniaxial mica, the majority of specimens of which have a negative or repulsive axis, though some have a positive or attractive one. But there is another kind of mica, of more frequent occurrence in the shops, and which is called by mineralogists prismatic or diaxial mica. It has two

* llose inserts "Palladium (?)" among rhombohedric crystals.

t Talc is readily distinguished from mica by its greasy or unctuous feel. The most familiar kind of talc is that sold in the shops under the name of French chalk. It is talc in an indurated earthy form.

axes of double refraction, and consequently when a plate of it is placed in the polariscope, two systems of coloured rings are perceived. This kind of mica exists in two forms; one is crystallized in right prisms, the other in oblique prisms. Hence I shall distinguish the one as right prismatic mica, the other as oblique pritmatic mica. They will be described hereafter. In conclusion, then, the kinds of mica maj be thus arranged:

rRhombohedric or Uniaxial {^^ST' IW— « Biaxial {*$$££*•

The principal constituents of mica are silica and alumina. But it also contains potash and sesquioxide of iron.

Properties.—The forms of this system possess four axesf; viz., three equal ones, called the secondary axes, placed in one plane, and crossing in the centre at an angle of 60°; and a fourth, termed the principal axis, or the axis of symmetry, or the crystallographical axis, perpendicular to the others, from which it differs in length. They are double refractors, with one optic axis coincident with the principal axis. They are di-unequiexpanding bodies, the expansion being different (greater or less) in the principal axis from that in the secondary ones. They are di-unequielastic; the elasticity in the principal axis being either more or less than that in the secondary axes. With regard to the atoms, we may assume their shape to be spheroids.

Iceland spar (Ca O. C02) may be conveniently used to illustrate the optical properties of the crystals of this system. It occurs in rhoml)oidal masses, which by cleavage yield obtuse rhombohedra. The line which joins the two obtuse summits of one of these rhombohedra, is called the shortest or principal axis, the crystallographical axis, the axis of the rhomboid, or simply the axis. A plane drawn through this axis, perpendicularly to a face of the crystal, is called the principal section. This section belongs rather to a face than to the entire crystal, for each face has its own. Now when the incident rays are perpendicular to the face of the crystal, both the ordinary and extraordinary rays are always found in the same plane, so that the deviation of the extraordinary pencil takes place in the plane of the principal section. Every plane in the interior of the crystal, which is perpendicular to the axis, is called a section perpendicular to the axis, or the equator of double refraction. In this plane the

+ The description adopted in the lectures is that of Weiss and Rose; some other writers admit only three axes. Thus, Turner {Elements of Chemistry 7th ed., p. 588) describes three equal but not rectangular axes; while Griffin {System of Crystallography, pp. 151 and 258) admits three rectangular but unequal axes. Neither of these modes of descriptions appear to me so completely to connect the form with the optical and other properties of the crystals, as Weiss and Rose's method.

[graphic][ocr errors]
« AnteriorContinuar »