On the production of regular double refraction in the molecules of bodies by simple pressure; with observations on the origin of the doubly refracting structure

Abstract

The author has already shown, in former papers which have ap­peared in the Philosophical Transactions, that the phenomena of double refraction may be produced artificially by effecting certain changes in the mechanical condition of hard and of soft bodies. In all these cases, he observes, the phenomena are entirely different from those of regular crystals; and in none of them is the doubly- refracting force a function of the angle which the incident ray forms with one or more axes given in position. In the year 1815, he no­ticed the depolarizing properties of a thin film of a mixture of resin and white wax, compressed between two pieces of glass. Acciden­tally meeting with the specimen which had originally been the sub­ject of this observation, he found that after fifteen years it still retained this property of depolarization, and was induced to pursue the inquiry to which it led. He varied the proportions of the ingredients, and observed in the different cases the modifications produced in the phenomena by employing various degrees of pressure. He found that, in every point, there existed an axis of double refraction, per­pendicular to the plane of the film, and that the doubly-refracting force varied with the inclination of the incident ray to this axis, just as happens with all regular uniaxal crystals. He infers from his observations, that the property of uniaxal double refraction is com­municated to the molecules simply by the agency of pressure; for in all cases where pressure has not operated, the aggregate does not exhibit this property. These effects are precisely the same as those which would be produced by subjecting elastic spheres to a regular compressing force, the axis of pressure becoming an axis of positive double refraction; while extension, on the contrary, produces a ne­ gative axis. From the consideration of the preceding facts, the author is led to a very simple explanation of the origin and general phenomena of double refraction in regular crystals. He considers this property as not being inherent in the molecules themselves, but as resulting from their compression, either by an extraneous force, or by their power of inherent attraction of aggregation. The phenomena of crystal­lization, and of cleavage, prove that the molecules of crystals have several axes of attraction, or lines, along which they are most power­fully attracted, and in the directions of which they cohere with dif­ferent degrees of force. Guided by the indications of hemitrope forms, and supposing the molecules to be spherical or spheroidal, it is inferred that these axes are three in number, and at right angles to each other, and that they are related in position to the geometri­cal axis of the primitive form. In like manner, the phenomena of double refraction are related to the same axis of the primitive form, and may be all rigorously calculated by a reference to three rectan­gular axes. The author pursues the consequences of these prin­ciples in their application to various kinds of crystals. It follows from this theory, that the forms of the ultimate molecules of crystals, existing separately, determine, within certain limits, the primitive form to which they belong; while the doubly-refracting structure, and the precise form of the crystal, are simultaneously produced by the action of the forces of aggregation. These views receive a remarkable confirmation in the doubly-refracting structure which the author discovered in chabasie; and they also enable us to under­stand the nature of that influence which heat produces on doubly-refracting crystals, as discovered by Prof. Mitscherlich. The optical phenomena exhibited by fluids under the influence of heat and pres­sure, and by crystals exposed to compressing or dilating forces, are also in perfect conformity with the above views, and would in them­selves have been sufficient to establish the principle that the forces of double refraction are not resident in the molecules themselves, but are the immediate result of those mechanical forces by which these molecules constitute solid bodies.