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

James Dwight Dana

Date:: 1847]

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

8/28 page 6

$kneaded together like a soft gum or jelly.” He concludes from the facts, that the topaz must have been in a soft and plastic state while it yielded to the compressing force which emanated from the cavities, and farther, infers (as in 1805 from his experiments on depolarization) “ the existence of a new species of crystalliza¬ tion, which is the effect of time alone, and which is produced by the slow action of corpuscular forces.” If the facts prove a comparatively soft state of the topaz when first formed, as they appear to do, they indicate that the crystal while afterwards hardening, would undergo contraction, in which case such minute cavities or any foreign particle would become apparent centres of a compressing force. The regular form of the topaz crystal, presenting a perfect correspondence in every respect with other crystals, can be explained only on the sup¬ position of axial attraction; and while the necessity of time and corpuscular forces must be admitted, we fail to see evidence of a new species of crystallization. In the diamond, the same prin¬ ciple may have operated. If the crystal when first formed is still imperfectly hardened, contraction would continue for a time in progress; and the contraction in this case acting with some reference to radii from the centre, and affecting most the prominent edges and angles, might give the convex form present¬ ed by the faces of the crystals, and the peculiar structure within. The fact mentioned at the meeting of the British Associa¬ tion in June last, by Mr. H. F. Talbot, that a fused globule of nitrate of potash polarized light, like a regular crystal, only shows that some species may retain the axial forces and arrange¬ ment in the fluid state. It is known that certain fluids have the power of polarization. These facts extend instead of limiting our proposition, proving that the molecules of some substances, even in the liquid state, may have the inequiaxal forms, generally detected only in solids.* * Sir David Brewster, after mentioning his grand discoveries with regard to producing the polarizing structure in glass and other substances by tension, sug¬ gests that the polarizing structure of crystals arises from the mutual pressure of the elastic particles during crystallization, lie concludes that the axes of crystal¬ lization are three in number and at right angles to each other, and observes, “that when any two molecules are brought together by the forces or polarities which produce a crystalline arrangement and strongly adhere, they will mutually com¬ press one another, and each will have an axis of double refraction in the direction of the line joining their centres, in the same manner as if they had been compress¬ ed by an external force.” The facts are explained, as we believe, more simply and more in accordance with the laws of crystals, by attributing the character of ordinary polarization in crystals to the form of the molecule independent of pressure. The molecule in ine¬ quiaxal crystals must be inequiaxal. and this is a sufficient reason, without adding pressure as an additional cause. Pressure at the poles, would only flatten the poles, and this would make an irregular solid of the ellipsoid, inconsistent with the$

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

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