Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . gles. Orthorhombic, the direction coincides with two of the crystalaxes. Monoclinic, one direction ftiay coincide with b, or both lie in theclino pinacoid. Triclinic, the axes have no fixed positions relative to crystal. The increase in expansion may be accurately measured for anydirection by the method of Fizeau, as follows : In Fig. 370 C is a plane parallelplate of the crystal, about 10 , which rests upon the steel plateT Three screws pass through T andsupport the glass plate P, which tape
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . gles. Orthorhombic, the direction coincides with two of the crystalaxes. Monoclinic, one direction ftiay coincide with b, or both lie in theclino pinacoid. Triclinic, the axes have no fixed positions relative to crystal. The increase in expansion may be accurately measured for anydirection by the method of Fizeau, as follows : In Fig. 370 C is a plane parallelplate of the crystal, about 10 , which rests upon the steel plateT Three screws pass through T andsupport the glass plate P, which tapersslightly. By adjusting the screws sothat the upper surface is horizontala thin wedge-like film of air is leftbetween the upper surface of the sub-stance and the lower surface of theglass. By a suitable arrangement, rays of monochromatic light strikethe apparatus normally. Rays incident at the upper surface of thecrystal and the lower surface of the glass wedge interfere on reflec-tion producing parallel dark bands wherever the thickness of the/. The distance between two adjacent bands is. wedge IS \^ 2 182 DESCRIPTIVE MINERALOGY. therefore a function of a wave-length of the light used and is meas-ured by a screw micrometer. When the apparatus is uniformly heated the crystal plate andthe interference apparatus both expand and the interference bandschange in distance apart. From this change and from the pre-viously determined effect of the expansion of the metal stand thechange due to the expansion of the crystal is calculated. Tutton has recently described an improvement of this apparatusin which the expansion of the metal stand is compensated by thatof a cylinder of aluminum. Change o*f Crystal Angles produced by Expansion. A crystal of the isometric system when uniformly heated ex-pands uniformly without change of angles. In all other systemsthe expansion varies with the direction and certain angles arechanged (sometimes several minutes for ioo° temperature altera-tion. For
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