Faraday As A Discoverer [30]
glass. It will not be doubly refracted. From the symmetry of the liquid figures, known to be produced in the planes of freezing, when radiant heat is sent through ice, we may safely infer symmetry of aggregation, and hence conclude that the line perpendicular to the planes of freezing is a line of no double refraction; that it is, in fact, the optic axis of the crystal. The same remark applies to the line joining the opposite blunt angles of a crystal of Iceland spar. The arrangement of the molecules round this line being symmetrical, the condition of the ether depending upon these molecules shares their symmetry; and there is, therefore, no reason why the wavelength should alter with the alteration of the azimuth round this line. Annealed glass has its molecules symmetrically arranged round every line that can be drawn through it; hence it is not doubly refractive. But let the substance be either squeezed or strained in one direction, the molecular symmetry, and with it the symmetry of the ether, is immediately destroyed and the glass becomes doubly refractive. Unequal heating produces the same effect. Thus mechanical strains reveal themselves by optical effects; and there is little doubt that in Faraday's experiment it is the magnetic strain that produces the rotation of the plane of polarization.[2] Footnotes to Chapter 10 [1] 'By a diamagnetic,' says Faraday, 'I mean a body through which lines of magnetic force are passing, and which does not by their action assume the usual magnetic state of iron or loadstone.' Faraday subsequently used this term in a different sense from that here given, as will immediately appear. [2] The power of double refraction conferred on the centre of a glass rod, when it is caused to sound the fundamental note due to its longitudinal vibration, and the absence of the same power in the case of vibrating air (enclosed in a glass organ-pipe), seems to be analogous to the presence and absence of Faraday's effect in the same two substances. Faraday never, to my knowledge, attempted to give, even in conversation, a picture of the molecular condition of his heavy glass when subjected to magnetic influence. In a mathematical investigation of the subject, published in the Proceedings of the Royal Society for 1856, Sir William Thomson arrives at the conclusion that the 'diamagnetic' is in a state of molecular rotation. Chapter 11. Discovery of diamagnetism--researches on magne-crystallic action. Faraday's next great step in discovery was announced in a memoir on the 'Magnetic Condition of all matter,' communicated to the Royal Society on December 18, 1845. One great source of his success was the employment of extraordinary power. As already stated, he never accepted a negative answer to an experiment until he had brought to bear upon it all the force at his command. He had over and over again tried steel magnets and ordinary electro-magnets on various substances, but without detecting anything different from the ordinary attraction exhibited by a few of them. Stronger coercion, however, developed a new action. Before the pole of an electro-magnet, he suspended a fragment of his famous heavy glass; and observed that when the magnet was powerfully excited the glass fairly retreated from the pole. It was a clear case of magnetic repulsion. He then suspended a bar of the glass between two poles; the bar retreated when the poles were excited, and set its length equatorially or at right angles to the line joining them. When an ordinary magnetic body was similarly suspended, it always set axially, that is, from pole to pole. Faraday called those bodies which were repelled by the poles of a magnet, diamagnetic bodies; using this term in a sense different from that in which he employed it in his memoir on the magnetization of light. The term magnetic he reserved for bodies which exhibited the ordinary attraction. He afterwards employed the term magnetic to cover the whole phenomena of attraction and repulsion, and used the word paramagnetic to designate such magnetic action as is exhibited