Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/395

Rh MINERALOGY 377 Mag netic polarity Heat conduc tivity. Senar- mont s investi gations. needle, and observe whether it causes it to vibrate. Another mode is&amp;gt; to apply a strong magnet to the mineral in powder. These are sufficient for the mineralogist. Dele.sse has experimented exten sively upon the magnetic force of minerals, and has determined the relative amount for numerous species. Calling this force for Styrian steel 100, the following are some of his results : Native platinum 2-173 to 3-047 Magnetic iron ore 15-00 to Co -00 Frankl :iite, from the United States 1-033 Chromic iron O lSfi to O OCS Spinel (pleonaste), from Monzoni, Tyrol 0-078 Titanic iron (rhombohedral), often magnetipolar 5-764 Specular iron, sometimes magnetipolar 0-14 to 2-35 Graphite 0-015 to 0-040 Spathic iron (spherosiderite, the highest) 0-092 to 0-287 Iron pyrites 0-0,&amp;gt;9 to 0-057 Vivianite 0-027 to 075 Columbite of Bodenmais and Iladdam 0-151 Pyrochlore O OIO Chrysoprase (quartz is diamagnetic, but many vane- ). nn( ties are magnetic) f Felspar, sometimes feebly magnetic. Labradorite of an antique green porpliyry 0-077 Hornblende 012 to 057 Crystallomag-netic Action. The magnetic polarity thus far alluded to belongs to the mass, and has no relation to crystalline form. There is also a kind oi polarity directly related to the crystalline or optic axes of minerals. A crystal of cyanite, suspended horizontally, points to the north, by the magnetic power of the earth only, and is a true compass needle, from which even the declination may be obtained ; and the line of direction is the line of the optic axes. Other crystals, which are called negative, take a transverse or equatorial position. The latter are diamagnetic crystals. Conductivity for Heat. Senarmont found that the con ducting power of colloids and of crystals of the cubic system is equal in all directions, but that it varies in different directions in crystals belonging to all the other systems, exhibiting characters analogous to those deduced from their double refraction, conformable with the optic axes of the crystal, and referable, as in the latter case, to axes of elasticity, or unequal compression of the molecules. The fundamental fact is easily shown by taking two slices of rock-crystal, one cut transverse to the axis and one parallel to it. Through the centre of each plate a small hole is drilled for the reception of a bent wire, which by insertion into the hole sustains the plate. The other end of the wire is to be heated, and the rate of the conduction of the heat is rendered visible by the amount of a thin coating of beeswax, with which the plate has been pre viously coated, which is melted round the central hole. It will be seen that in the transverse slice the wax is melted in a circular form, while in the longitudinal slice the form is elliptical (fig. 256). The conduction is equal in all directions, as regards the transverse axes of the hexagonal prism, but more rapid in one direction in the longitudinal slice, and that direction is the line of its optic axis. In the case of quartz the two diameters of the ellipse are as 1000 to 1312. If the regular disposition of the molecules of amorphous bodies be interfered with by unequal tension or compression, the regularity of their power of conducting heat is destroyed, and they also show elliptical forms of melted wax ; and the shorter axis of the ellipse is in the line of pressure or undue packing of the molecules. The heat thus does not travel so fast in this direction, partly because it is spent in the heating up of the greater number of molecules. Hence we might conclude that along the main axis of quartz a smaller number of molecules are packed in an equal space than along the transverse. The following are the more important of Senannont s results. L Crystals of the tetragonal and rhombohedral systems have one axis of conductivity which is either greater or smaller than the others, and this axis coincides with the main crystallographic axis. The isothermal surfaces are ellipses which lie in the line of this axis, and these ellipses may be either elongated or flattened in the direction of this line. 2. In crystals of the right prismatic system the isothermal surfaces have three unequal axes, which coincide with crystallo graphic axes drawn parallel to the edges of the rectangular prism. 3. In crystals of the oblique rhombic system the isothermal Fig. 256. surfaces have three unequal axes, one of which coincides with the horizontal diagonal of the base, while the other two have directions which are not referable to any law. 4. In crystals of the anorthic system the isothermal surfaces have three unequal axes, all with indeterminable positions. In crystals of a single axis there appears to exist no constant relation between the axis of optic elasticity, whether maximum or minimum, and the axis of the greatest or of the least calorific conductibility. Thus, of the minerals examined by Senarmont, quartz ( + ), calcite ( - ), cassiterite ( + ), rutile ( + ), and calomel ( + ) have all their greatest axis of conductibility parallel to the principal axis ; idocrase, beryl, tourmaline, and corundum, all optically negative, have on the contrary their smallest axis of conductibility parallel to the axis. In crystals belonging to the oblique rhombic system there is rarely coincidence between the thermic axes and the axes of optic elasticity. In gypsum and in felspar these lie apart to a marked extent. Dilatation by Heat. In crystals of those systems in which Dilata- the molecules are arranged unequally as regards their axes, tion - the amount of their dilatation when heated is unequal in the direction of their axes. Our knowledge of this subject is chiefly due to Mitscherlich. In crystals of cubic symmetry the expansion is equal in all directions. The dimetric systems the pyramidal and hexagonal are brought together as regards this quality, inasmuch as the axes of volumetric change are in these the same ; for, while these in the pyramidal correspond with the crystallographic axes, in the hexagonal the three axes are the vertical, one lateral axis, and an axis lying intermediate to the other two and at right angles to the first lateral axis. The expansion along the principal axis may be either greater or less than along the others ; and in some minerals there is even contraction along one axis. In the right prismatic system the axes of dilatation correspond to those of form. In the oblique prismatic one axis corresponds with the orthodiagonal, but the others make angles not only with the other crystallographic axes but. strange to say, with the axes both of thermic conductivity and of optic elasticity. We arc as yet ignorant of the properties of anorthic crystals in this respect. As a consequence of this unequal expansion along different axes, the angles of crystals, other than those of the cubic system, are altered under the influence of heat. The alteration is extreme in the case of calcite, where, through elongation along the vertical axis, with some concomitant contraction of the transverse, the angle of the rhombohedric faces is, when the crystal is heated from 32 to 212 F., diminished from 105 5 to 104 56 23&quot;, the form thus approaching that of a cube, as the temperature is raised. Dolomite, in the same range of temperature, diminishes 4 46&quot;. In some rhombohedrons, as of calc-spar, the vertical axis is lengthened (and the lateral shortened), while in others, like quartz, the reverse is true. The variation is such, either way, that the double refrac tion is diminished with the increase of heat ; for calc-spar possesses negative double refraction, and quartz positive. According to Fresnel the same is true of gypsum. The dilatation for calc-spar, according to experiment, is OOJ961. Kopp has shown that in the carbonates of lime, magnesia, iron, manganese, and zinc, which are nearly the same in the angle of their crystals, the vertical axis is shorter the greater the atomic volume And since heat diminishes the density, and therefore necessarily increases the volume, the axis a should be lengthened by an increase of temperature, as is actually the case. He has determined by cal culation that the change of angle from 32 to 212 should be 7 37&quot;. Although in the greater number of cases the variations are so small as to be scarcely measurable, yet they may be sufficient for establishing a difference between substances which have identical geometric form while belonging to different systems of crystalliza tion. The angle of arhombohedron might at a certain temperature be 90, and so coincide with a cube ; but that angle would in a rhombohcdron change whenever the temperature altered, while the angle of a true monometric cube is constant at all temperatures. The increase in volume and diminution in density which generally result from heating are always accompanied by a change in optical properties. In trimetric crystals, where the principal indices alter unequally, the change affects the amount of divergenre of the optic axes. The amount of alteration in gypsum, when the divergence is diminished, is extreme. __ At the ordinary temperature the angle of the divergence of the optic axes which lie in the plane of symmetry is about 90 for red light ; when heated to 177 it is diminished to 0, and for the moment the crystal appears to be uniaxal. When more highly heated, the axes again diverge, but in a plane at right angles to the original one, and in cooling these changes take place in reverse order. In barytes and celestine again, the alteration in the angle of the optic axes is a divergence when heated. XVL 48