Uniaxial Crystals give their most character istic interference figure when the section is at right angles to the optic axis. This figure con ,ists of a dark cross, the arms of which, called the intersect in the centre of the field and remain parallel to the vibration directions of the nicols during rotation of the fragments. It is due to those rays which then have their vibration planes parallel to the nicols. As the stage is rotated successive rays come into these positions, maintaining the same effect.
With monochromatic light the field will be the color used but if the section is not too thin, the centre of the black cross will he sur rounded by concentric dark circles, correspond ing to rays for which P= A, 2A, 3?,, etc.
With white light the concentric circles will he color rings, arranged strictly in the order of the interference color.
Biaxial Crystals yield characteristic interfer ence figures both with sections normal to an optic axis and with sections normal to the bi sectrics X and Z. In the former the isogyre is a single black bar, essentially straight, when the trace of the plane of the optic axes coincides with the vibration direction of either nicol, but for all other angles of rotation it is curved and resembles one arm of an hyperbola. This arm rotates in the opposite direction to the rotation of the stage.
If the section is not too thin there will be black and often nearly circular interference bands with monochromatic light, and colored curves with white light corresponding to Al_ X, 2X, 3X, etc.
Sections normal to a bisectrix yield an inter ference figure in which the ccisogyres° appear as two dark bars or brushes. For the so-called normal position (Fig. 8) one bar connects the points of emergence of the optic axes, the other is a thicker, lighter band at right angles to the first and midway between the axes.
If the stage is rotated the straight bars seem to dissolve into an hyperbola, the vertices of which are the loci of the optic axes and the branches of which rotate in the opposite direc tion to the rotation of the stage.
With convergent monochromatic light there will be, in a field of the color used, black closed curves around the loci of the optic axes cor responding to A-= X, 2X, 31, etc. If white light is used isochromatic curves result, but the interference figure may be much more complex than with monochromatic light as neither the axial loci nor the isogyres nor the curves of equal retardation coincide for different wave lengths. The angle between the optic axes may be obtained from the last•described interference figure.
Under favorable conditions interference figures obtained with white light will serve to distinguish between the orthorhombic, mono clinic and triclinic systems, for they conform in shape and distribution of color to the sym metry of the system. With orthorhombic crystals the figure will be symmetrical to the line joining the optic axes, to the line through the centre at right angles thereto and to the central point, with monoclinic crystals the figure will be symmetrical to only one of these and with triclinic crystals to none.
Interference figures furnish a ready means for ascertaining whether the ray surface is positive or negative.
Thermal Properties of Heat rays differ from light rays in their relatively greater length, but may be reflected, refracted, doubly refracted, polarized and absorbed, and it is possible, though difficult, to determine a series of thermal constants for crystals.
conduct heat with unequal rapidity in different directions. If a crystal face or cleavage surface is coated with an easily melted wax and touched with the point of a hot wire the wax will melt in a circle wish either an isometric crystal or a basal section of a hexagonal or tetragonal crystal. All other sections will ellipses varying in eccentricity and in position of acres.
When a crystal is uniformly heated, directions crystallographically alike ex pand in the same proportion, but directions un like do not. This is shown by the fact that cer tain interfacial angles are changed, others are not, and the expansion may be accurately meas ured for any direction, but the methods in volve apparatus of great precision and cost.
The Magnetic Properties of Crystals.— All crystals and indeed all substances are to some extent either attracted or repelled when placed in the field of a powerful electro magnet. Those that are attracted are para magnetic, those repelled, diamagnetic. The strength of the magnetization varies with the direction in crystals, and so far as studied the magnetic relations are analogous to the optical relations with the curious exception that the isometric crystals of magnetite are not mag netically isotropic, but show different magnetic intensity in different directions.
Electrical Properties of waves are like light waves; they travel with the same velocity, exhibit the phenomena of reflection, refraction and polarization and differ only in much greater length. Bose has ix scribed an electric polariscope, with which it may be possible to test opaque crystals as we now test transparent crystals.
Electric In the few tested crystals there has been found a dependence of electric conductivity upon crystallographic direc tion, conforming to the thermal conductivity.
of temperature will develop electric charges in certain classes of crystals. Usually the crystal is heated and al lowed to cool. During the cooling of the crystal positive charges collect at the so-called antilogue pole and the negative charges at the analogue pole.
Piezoelectricity.—Similar charges are de veloped by pressure, for instance tourmaline compressed in the direction of the vertical axis develops a positive charge at the antilogue end and a negative charge at the analogue end, or precisely the charges which would result from cooling a heated crystal.
Most of larger textbooks of mineralogy and special works such as v. Groth, P., (Physikalische Krystallographie' (Leipzig, also translation of part by Jackson, New York 1910) ; Becker, A., 'Krystalloptik F. Enke' (Stuttgart 1903) ; Fletcher, L., The Optical indicatrix' (London 1892) ; Tamman, G., and (Leipzig 1903) ; Voigt, W., (Elemente der Krystallphysik' (Leipzig 1898).