Text: In 1822 Poisson predicted that a body possessing crystalline structure would, if magnetic at all, have different magnetic powers in different directions. In 1847 Plucker discovered that a piece of tourmaline, which is itself feebly paramagnetic, behaved as a diamagnetic body when so hung that the axis of the crystal was horizontal. Faraday, repeating the experiment with a crystal of bismuth, found that it tended to point with its axis of crystallization along the lines of the field axially. The magnetic force acting thus upon crystals by virtue of their possessing a certain structure he named magne-crystallic force. Plucker endeavored to connect the magne-crystallic behaviour of crystals with their optical behaviour, giving the following law: there will be either repulsion or attraction of the optic axis (or, in the case of bi-axial crystals, of both optic axes) by the poles of a magnet; and if the crystal is a "negative" one (i.e., optically negative, having an extraordinary index of refraction less than its ordinary index) there will be repulsion, if a "positive" one there will be attraction. Tyndall has endeavored to show that this law is insufficient in not taking into account the paramagnetic or diamagnetic powers of the substance as a whole. He finds that the magne-crystallic axis of bodies is in general an axis of greatest density, and that if the mass itself be paramagnetic this axis will point axially; if diamagnetic, equatorially. In bodies which, like slate and many crystals possess cleavage, the planes of cleavage are usually at right angles to the magne-crystallic axis. Another way of stating the facts is to say that in non-isotropic bodies the induced magnetic lines do not necessarily run in the same direction as the lines of the impressed magnetic field.

See Also: PARAMAGNETISM; OXYGEN

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