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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    form; and if the prism has oblique angles instead of being rect¬ angular, these lines of strongest attraction must have a corres¬ ponding obliquity. Hence the angles referred to, as characteri¬ zing cohesive attraction, are angles between certain imaginary lines, or axes, in lohose direction the attraction is strongest. Again : the crystalline forms in nature are well known to have fundamentally fixed relative dimensions, indicated by the modi¬ fications they undergo (producing secondary forms) though not necessarily apparent in the actual proportions of the crystal. Thus a cube, which is equal in its dimensions, shows it in all its modifications; and in a prism, the inequality in the dimensions is as exactly and precisely indicated by its modifications A The relative dimensions belonging to the fundamental form of a sub¬ stance, are often therefore easily calculated; and the whole sci¬ ence of crystals is thus based on rigorous mathematical laws. From these facts we may conclude therefore with respect to solids, that Cohesive Attraction is characterized by fixed angles, as regards the direction of its action, and by specific relations of force in certain axial directions; and it differs in these par¬ ticulars for different substances.f These facts are the only hints which nature gives us respecting the axial dimensions of molecules. We proceed on the only possi¬ ble grounds for any conclusion on this point, when we infer that molecules have corresponding relative dimensions with the crys¬ talline forms, and the same specific angles between the funda- * A square prism and a cube as presented in nature, may have actually the same dimensions, owing to the distortion of the one or the other. But the funda¬ mental nature of their forms, may, notwithstanding, be obvious to the eye. In the cube (having equal faces and axes) all the edges will have similar secondary planes (that is, these planes will be identical in their inclinations to the faces of the cube) ; in the prism, the secondary planes of the lateral edges will differ in their inclinations from those of the terminal. In the cube, there may be a plane on any edge equally inclined to the faces of the cube. In the prism, the plane on a terminal edge will always be unequally inclined to the including faces; such a plane removes a part of the two including faces, and these parts will in all cases have a definite relation to the height and breadth of the prism. These explana¬ tions are sufficient to render the general principle stated, intelligible to those unac¬ quainted with the structure of crystals. Treatises on crystals should be studied for a full exposition of this subject. t Apparent exceptions to this principle are supposed to exist in the case of glass, tabasheer and some other substances tiiat do not polarize light. They are, how¬ ever, not necessarily exceptions. For in these substances the molecules are either cquiaxal or inequiaxal ;—if inequiaxal, the solids prove that such molecules, with weak polar forces, may be so irregularly aggregated as to show no polarization; and if their form is equiaxal, no rings of polarization are to be expected. We have no facts to determine the form of the molecules in glass. As glass, (common as well as volcanic,) when slowly cooled, produces a different material, with a different temperature of fusion, it must be a dimorphous or isomerous mate¬ rial, capable of taking two distinct forms, of which glass is one. We cannot, therefore, infer that tide molecules of glass are inequiaxal from the forms of the crystallized silicates of an alkali. The rapid cooling, necessary to form glass, is the most unfavorable for an axial arrangement of the particles, even if the polar forces were strong.