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Crystals. The simple or elementary bodies which crystallize in forms belonging to this system are only three, namely, iodine, native sulphur, and selenium.

Among the binary compounds we have pyrolusite (binoxide of manganese), white antimony (sesquioxide of antimony), bichloride of mercury, chloride of barium, orpiment, and grey antimony (sesquisulphuret of antimony).

A considerable number of salts belong to this system, as the carbonates of lead, baryta, strontian, potash, and ammonia; the bicarbonate of ammonia, and that variety of carbonate of lime called arragonite; the nitrates of potash, ammonia, and silver; the sulphates of magnesia, zinc, baryta, and strontian, and bisulphate of potash; Rochelle salt (tartrate of potash and soda) and emetic tartar (tartrate of potash and antimony).

To the above must be added the following substances: topaz, dichroite, citric acid and morphia.

Properties. The crystals of this system present the following properties: they have three rectangular axes all of different lengths they are doubly refracting with two optic axes; and are tri-unequiexpanding. Consequently they have three rectangular unequal elasticities. On the ellipsoidal hypothesis, their atoms are ellipsoids, with three unequal axes.

They present no crystallographical character by which the principal axis can be distinguished from the others called secondary axes; so that in a geometrical point of view the choice of this axis is altogether arbitrary. But considered optically the principal axis is the middle point between the two nearest poles of no polarization. It corresponds with what is called by Mr. Brooke the prismatic axis; that is, the axis which passes through the centres of the terminal planes of the prism.

If you examine one of the simple or primary forms of this system-say this unmodified rectangular prism (the outer prism of figure 39), you observe there is no single line around which the figure is symmetrical; nor any square plane, or plane which can be inscribed within the circle. But let each of the two opposite terminal edges be replaced by a square plane, both equally inclined to the prismatic axis, and the line which passes through the centre of each of these planes will represent the direction of one of the optic axes.

As the crystals of this system have two optic axes, they present, when examined by the polariscope, a double system of rings. In nitre, carbonate of lead, and arragonite the inclination of these axes is small; and, therefore, both systems of rings may be seen at the same time.

In order to examine these by the polariscope, we must, in the case of the three crystals just mentioned, cut slices of them

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perpendicularly to the principal or prismatic axis. But in topaz, right prismatic mica, and Rochelle salt, the inclination of the optic axes is too great to permit both of them to be seen simultaneously; and, therefore, only one of them can be seen at a time. Consequently if we examine, by the polariscope, a plate of any of these crystals, cut at right angles to the prismatic axis, we must incline it first on one side and then on the other, to see successively the two systems of rings. To obviate this inconvenience, plates of these crystals should be prepared by grinding and polishing two parallel faces perpendicular to the axis of one system of rings.

Nitrate of potash, also called nitre or saltpetre, (NO, + KO) is a very instructive crystal for illustrating the double system of rings. It is usually met with in the form of a six-sided prism, with diedral summits.

axis.

For placing in the polariscope, we use plates of from th to th of an inch in thickness, cut perpendicular to the prismatic If one of these be put in the polariscope in such a position that the plane passing through the optic axis is in the plane of primitive polarization, we shall then perceive a double system of coloured elliptical or oval rings, intersected by a cross, but the centre of the cross is equidistant from the centres of the two systems of rings, so that through the centre of each system passes one arm or bar of the cross, the other arm being at right angles to the former. When the polarizing and analyzing plates are crossed, we have a double system of coloured rings, with a black cross (fig. 40); but when the polarizing and analyzing plates coincide, we have another double system of coloured rings, exactly complementary to the first, with a white cross (fig. 41).

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If when the analyzing and polarizing tourmaline plates are crossed, we revolve the plate of nitre in its own plane (both the tourmaline plates remaining unmoved) the black cross opens into two black hyperbolic curves. When the angle of rotation is a quarter of a right angle, we have the appearance represented by

(fig. 42); when it equals half of a right angle, the black arms have assumed the forms of fig. 43.

[blocks in formation]

DC DC

Here, then, is a remarkable distinction between biaxial and uniaxial crystals, for you will remember I demonstrated that when the uniaxial crystal was rotated in the polariscope, the black cross retained its position and shape.

The variation of form, as well as the general figure of the isochromatic lines, resembles the curve called by geometers the lemniscate. The inner rings encircle one pole only, but the outer ones surround both poles. The number of rings which surround both poles augments, as we diminish the thickness of the plate of nitre, until all the rings surround both poles, and the system thus greatly resembles, in appearance, the rings of an uniaxial crystal, from which, however, they are distinguished by their oval form.

I have already stated, that in biaxial crystals the optic axes. for different colours do not coincide. In the case of nitre, the axes for red make with each other a smaller angle than the axes for blue. Hence the red ends of the rings are inward, that is, between or within the two optic axes, while the blue ends are outwards, or exterior to the two axes. But as the red rings are larger than the blue ones, it follows that there are points exterior to the axes where all the colours are mixed, or all are absent. At these spots, therefore, the rings are nearly white and black. Now if we trace the same rings to the positions between the axes, "the red rings will very much over-shoot the blue rings; and, therefore, the rings have the colour peculiar perhaps to a high order in Newton's scale *."

Native crystallized carbonate of lead constitutes a splendid polariscope object. It is to be cut like nitre; that is, perpendicularly to the prismatic axis. The optic axes are but slightly inclined (about 103°) and, therefore, both of them may be simultaneously perceived. The systems of rings have a similar form

* Airy, Mathematical Tracts, p. 396. 2d ed. 1831.

to those of nitre, and like the latter, the red ends of the rings are inwards, the blue ends outwards.

Arragonite forms an interesting polariscope object. It is identical in chemical composition with calcareous or Iceland spar, but differs in crystalline form: calcareous spar belonging to the rhombohedric, arragonite to the right prismatic, system. According to Gustav Rose, both these forms of carbonate of lime may be artificially produced in the humid way, but calcareous spar at a lower, arragonite at a higher, temperature. In the dry way, however, calcareous spar alone can be formed.

The inclination of the optic axes of arragonite being small (about 18°) we can easily see, at the same time, the two negative systems of rings surrounding their two poles, but considerably more separated than in the case of nitre. For this purpose, a plate of the crystal is to be cut perpendicularly to the prismatic axis, that is, equally inclined (at about 9°) on each of the optic axes. If we rotate the plate of arragonite on its axis in the polariscope, the tourmaline plates being crossed and unmoved, the two sets of rings appear to revolve around each other. By superposing two plates of arragonite, we obtain four systems of rings.

In Rochelle salt (tartrate of potash and soda) the optic axes of the differently refrangible or coloured rays are considerably separated. If a plate of this crystal, cut perpendicularly to the prismatic axis, be inclined first on one side and then on the other, both the systems of rings may be successively perceived. But to observe the separation of the axes for differently coloured rays, Sir J. Herschel directs the plate to be cut perpendicularly to one of its optic axes. If we view the rings with homogeneous light they appear to have a perfect regularity of form, and to be remarkably well defined. With differently coloured lights, however, they not only differ in size but in position. If the light be "alternately altered from red to violet, and back again, the pole, with the rings about it, will also move backwards and forwards, vibrating, as it were, over a considerable space. If homogeneous rays of two colours be thrown at once on the lens, two sets of rings will be seen, having their centres more or less distant, and their magnitudes more or less different, according to the difference of refrangibility of the two species of light employed."

Topaz (a fluosilicate of alumina) belongs to this system. As the inclination of its optic axes is great (about 50°), we can see at once only one of its two system of rings. It slits with facility in planes perpendicular to its prismatic axis, and equally inclined to its two optic axes. If we take a plate cut perpendicularly to

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the prismatic axis, and incline it first on one side and then on the other, we shall see successively two systems of oval rings, which have been very elaborately described by Dr. Brewster.

The plates of topaz sold in the opticians' shops, for polariscope purposes, have been obtained by cutting the crystal perpendicularly to one of the optic axes; that is, at an angle of about 25° to the prismatic axis. With these we only see one system of nearly circular rings traversed by a bar or arm of the cross. We observe also, that the optic axes for different colours are somewhat separated; for the red ends of the rings are inwards, or within the resultant axes, while the blue ends are outwards.

The topazes, which are cut for optical purposes, come from Australia, and are technically known as Nova Minas. They are colourless and remarkably free from flaws and macles.

Exceptions. In this system, as in the others, we meet with exceptions to some of the statements above made.

1. Macled crystals, especially of Nitre and Arragonite, are very common. Occasionally idiocyclophanous crystals of nitre are met with. These will be noticed subsequently.

2. Sulphate of potash is a tesselated or composite crystal, and as such will be described hereafter.

3. Some specimens of Brazilian topaz are tesselated.

SYSTEM V.

OBLIQUE PRISMATIC SYSTEM.

Synonymes.-The two- and one-membered system, the hemiorthotype system, the monoklinohedric system, or the hemihedricrhombic system.

Forms. To this system belong the oblique octohedron with a rectangular base, the oblique rectangular prism, the oblique octohedron with a rhombic base, and the oblique rhombic prism. Mr. Brooke's right oblique-angled prism is referred to this system.

Rose makes no distinction between the homohedral and hemi.. hedral forms in this system; but enumerates the following as the forms of the system:

1. Forms whose faces are inclined to all the three axes (Octohedra). 2. Forms whose faces are inclined to two axes, and are parallel to the third axis (Prisms).

3. Forms of which the faces are inclined towards one axis and parallel to the two others.

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