On certain laws of cohesive attraction / By James D. Dana.

  • James Dwight Dana
Date:
1847]
Catalogue details

Licence: Public Domain Mark

Credit: On certain laws of cohesive attraction / By James D. Dana. Source: Wellcome Collection.

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    axes, as these are only imaginary lines of concentration of force ; the other parts of the molecule must necessarily have attracting force though to a less amount than along the axial lines.* The fact that crystals are formed by the superposition of mole¬ cules by axial attraction, is a matter of observation. In an evap¬ orating brine we may see the minute cube of salt enlarging with¬ out change of form, a fact which implies that ranges of particles are added regularly to each side. In a drop of sea water under the microscope, we may watch the growing crystal of gypsum, and see its rhombic and arrow-head forms as perfect in the small¬ est visible point, as afterwards when more enlarged • proving again that the particles are added in fixed lines, since in no other way could there be this constancy of angle. It is proved again by finding many instances in calc spar, quartz and other minerals, of crystals with internal layers of another mineral which were deposited on the faces of the crystal during an intermission in their progress; showing the form of the crystal in its earlier stages. Hence we may not doubt the reality of the axial lines of cohesive attraction. Brewster, in the course of his splendid researches on the opti¬ cal phenomena of crystals, has shown that in some instances the particles are in a state of tension, as by compression. In a re¬ cent article on the topaz,f he describes the occurrence, in certain crystals, of extremely minute cavities, which indicate by means of polarized light, that the parts adjoining have been acted upon by a compressing force. Long since he observed respecting the diamond that its crystals,—which are peculiar in having convex faces,—exhibit, as he states, “an imperfect, doubly-refracting structure, as if aggregated by irregular forces, and compressed or * The several axial conditions illustrated in crystals include all the possible va¬ riations of the three diameters of spheroids, as is mentioned by the author in an article in this Journal, vol. xxx, 1836, p. 282, and Mineralogy, 2nd edition, p. 79. They are as follows, (using the term axes for the diameters having rectangular intersections; and diameters, for the diameters having oblique intersections.) 1. Sphere—Three conjugate axes ; equal, (1.) Cube. II. Ellipsoid, of revolution. A. Three conjugate axes, the two lateral equal, (2.) Right square prism. B. Three equal conjugate diameters, [with equal oblique angles of intersection,] (3.) Rhombohedron. III. Ellipsoid, not of revolution. A. Three conjugate axes, unequal, (4.) Right rectangular prism. B. A vertical axis, and two equal conju¬ gate diameters, (5.) Right rhombic prism. C. A vertical axis, and two unequal con¬ jugate diameters, (6.) Right rhomboida,l prism. D. Th ree conjugate diameters, two equal, (7.) Oblique rhombic prism. E. Three conjugate diameters, unequal, (8.) Oblique rhomboidal j)rism. f L. E. and D. Phil. Mag., August, 1847, xxxi, 101.
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