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

  • Dana, James Dwight, 1813-1895.
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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    come thus geniculated simultaneously at both extremities, while they were enlarging, evincing that— IV. The 'polarity of molecules may be reversed by extrinsic in¬ fluence* 5. Twin crystals have one half of the crystal in a reverse po¬ sition from the other, f This may be imitated by cutting a crys¬ tal in halves, and after a semirevolution of one half, applying the parts again together; an oblique crystal will thus have a reen¬ tering angle, as in fig. 5. If in fig. 5, A be the position of the nucleal molecule for one half, B must be that for the other. The mode of aggregation for a simple crystal is that shown in fig. 4. The attrac¬ tion between the two molecules brought them together in either case. But in one the position of the molecules when about uniting was favorable for the direct union in fig. 4; * and in the other, the two were in opposite positions, yet so close in proximity that union took place by the adjacent poles without allowing of the change of position necessary for direct union. In the former, the same poles of the vertical axes, are in the same direction; and in the latter, they are in opposite directions. There could not be such an inversion of the molecules, if the axes were a result of the act of union. Hence,— V. The axes and polarity of cohesive attraction in solidifica¬ tion exist before the union of the molecules, instead of being a consequence of that union. 6. The forms of inequiaxal crystals vary somewhat with a change of temperature; and at certain temperatures, specific in each case, some substances undergo abruptly a total change of form, both as to the direction and relative lengths of the axes. Hence,— VI. The axial lines of cohesive attraction, are not indefinitely fixed in position, but are some way modified in direction and force by temperature. Thus far we have considered the general polar condition of co¬ hesive attraction in solidification, its liability to a reversion of the Fig. 4. Fig. 5. * See an article by the author, in the American Journal of Science, for 1836, vol. xxx, p. 275, and particularly p. 292; also Mineralogy, the chapter on Crys- tallogeny. t We consider here but A single kind of twin crystals, in the briefest manner possible, as the facts are sufficient for our deductions. See farther, Amer. Jour, of Science, and Mineralogy, as just referred to.