Elements of Crystallography; After the Method of Hauy With, or Without Series of Geometrical Models, Both Solid and Dissected Exhibiting the Forms of
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1813 edition. Excerpt: ...result right prisms with three planes, as may be seen by inspecting Fig. 66, which represents one of the bases of the hexahedral prism divided into small equilateral triangles, which are the bases of so many moleculae, and which, being taken two and two, as A ip p h, C IG r, &c. form quadrilateral prisms with rhombuses for their basest By adopting then the tetrahedron in the doubtful case, we reduce, in general, all forms of integral moleculae to three, remarkable for their simplicity; viz. the parallelopipedon, which includes the cube; the triangular prism; and the tetrahedron. This simplicity may furnish a reason for the preference given to the tetrahedron in fluor spar, and the other substances of which we have spoken. The Abbe Haiiy, however, forbears deciding on this subject, as the want of accurate and precise observations leaves to theory nothing but conjectures and probabilities. But the essential object is, that the different forms to which the mixt structures in question conduct, are assorted in such a manner, that their assemblage is equivalent to a sum of small parallelopipedons, as we have seen to be the case in regard to fluor spar; and that the laminae of superposition, applied on the nucleus, decrease by subtractions of one or more ranges of these parallelopipedons; so that the basis of the theory exists independently of the choice which might be made of any of the forms obtained by the mechanical division. By the help of this result, the decrements to which crystals are subject, whatever be their primitive forms, are found brought back to those which take place in substances where this form, as well as that of the moleculas, are indivisible parallelopipedons; and theory has the advantage of being able to generalize its...
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