A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . of the cube (6) give 48 spaces. These are shown ina crystal form in a hexoctahedron (Fig. 31) of w^hich the limitform is obviously the cube. There are other limit forms as therhombicdodecahedron, tetrahexahedron, octahedron, trigonal tri-soctahedron, and tetragonal trisoctahedron, or, all the holohedralforms of the isometric system. It makes an excellent practice forthe teacher to take Prof. Moses book, and, throughout the classesof sym
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . of the cube (6) give 48 spaces. These are shown ina crystal form in a hexoctahedron (Fig. 31) of w^hich the limitform is obviously the cube. There are other limit forms as therhombicdodecahedron, tetrahexahedron, octahedron, trigonal tri-soctahedron, and tetragonal trisoctahedron, or, all the holohedralforms of the isometric system. It makes an excellent practice forthe teacher to take Prof. Moses book, and, throughout the classesof symmetry given by him, show how the limit forms arise fromthe general form in each class of symmetry. This can be done inthe isometric forms with a more or less flexible model which canbe compressed or elongated, and which initially, for the 32d class,shows the hexoctahedron with its six faces in each octant. In the hexagonal system, the twenty-seventh class of sym-metry is illustrated in the general form of the Dihexagonal Pyra-mid which, dissected by six vertical planes, and one horizontalplane, gives twenty-four faces; of which again the limit forms are. CO unj N o ^ o ^ (J C o •^ U CO NH < O e?l u 23 DEFINITION OF TERMS 51 the basal pinacoid, hexagonal prism of the first order, hexagonalprism of the second order, dihexagonal prism, hexagonal pyramidof the first order, hexagonal pyramid of the second order, or allthe holohedral forms of the hexagonal system. It has been felt that the reference of crystal forms to classesof symmetry is preferable to their reference strictly to rigid systemswhich are now more regarded as artificial conveniences. It hasbeen said (Lewis) that the idea of menohedrism (hemihedrism,etc.), leads to inconsistencies, and to representations of the crystalswhich are not in accordance with the facts. In this popular and general presentation of mineralogical datathe Naumann nomenclature, formulas, and conceptions (hemihe-drism, tetartohedrism, etc.), ha
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
