An introduction to the study of minerals : with a guide to the mineral gallery / by L. Fletcher.

  • British Museum (Natural History). Department of Mineralogy
Date:
1914
Catalogue details

Licence: In copyright

Credit: An introduction to the study of minerals : with a guide to the mineral gallery / by L. Fletcher. Source: Wellcome Collection.

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    Invention of axes and of invention of 36. Weiss,* the Professor of Mineralogy at Berlin, was the Systems of first to invent a mode of treatment which connected together the crystallisa- facts without requiring the assistance of any theory of structure tion by Weiss and at al]- Mohs inde- In the first place, he arranged the primitive forms of ' Haiiy into four classes, each distinguished by a purely geo- metrical character. I. By joining the centres of the opposite faces of the cube, or the opposite solid angles of the regular octahedron, or the three pairs of similar solid angles of the rhombic dodecahedron, or the middle points of opposite edges of the regular tetrahedron, he* obtained in each case three equal lines at right angles to each other. II. Similarly, from the octahedron or the right prism with a square base, he again obtained three lines at right angles, but now only two of them were of equal length. III. From a rhombohedron, or a regular six-sided prism, or a double six-sided pyramid, he obtained three lines in the same plane, all equal in length and equally inclined to each other, and a fourth line differing from the others in length and having a direction perpendi- cular to their plane. IV. From an octahedron or a four-sided prism not having a square base, he obtained three lines at right angles but all of different lengths. 37. Conversely, starting from these four classes of sets of lines, Weiss deduced all the primitive forms of Haiiy by con- structing planes which passed :— i. —through ends of three lines, ii. —through ends of two of the lines and parallel to a third, or iii.—through an end of one of the lines and parallel to two of them. * De indagando formarum crystallinarum charactore geometrico principali dissertatio. Lipsise, 1809. Uehersiehtliche Darstellung der verschiedenen natiirlichen Abtheilungen der Krystallisations-sj steme. (Denksch. d. Berl. Ak. d. Wissensch. 1814-15.)
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