to ordinary induced magnetism in all respects, except that the direction of the induced polarity is reversed. It was accepted by other investigators, notably by W. Weber, Plücker, Reich, and Tyndall; but was afterwards displaced from the favour of its inventor by another conception, more agreeable to his peculiar views on the nature of the magnetic field. In this second hypothesis, Faraday supposed an ordinary magnetic or para-magnetic[1] body to be one which offers a specially easy passage to lines of magnetic force, so that they tend to crowd into it in preference to other bodies; while he supposed a dia- magnetic body to have a low degree of conducting power for the lines of force, so that they tend to avoid it. "If, then," he reasoned,[2] "a medium having a certain conducting power occupy the magnetic field, and then a portion of another medium or substance be placed in the field having a greater conducting power, the latter will tend to draw up towards the place of greatest force, displacing the former." There is an electrostatic effect to which this is quite analogous ; a charged body attracts a body whose specific inductive capacity is greater than that of the surrounding medium, and repels a body whose specific inductive capacity is less; in either case the tendency is to afford the path of best conductance to the lines of force.[3]
For some time the advocates of the "polarity" and "conduction" theories of diamagnetism carried on a contro- versy which, indeed, like the controversy between the adherents of the one-fluid and two-fluid theories of electricity, persisted after it had been shown that the rival hypotheses were mathe- matically equivalent, and that no experiment could be suggested which would distinguish between them.
Meanwhile new properties of magnetizable bodies were being discovered. In 1847 Julius Plücker (b. 1801, d. 1868), Professor of Natural Philosophy in the University of Bonn, while repeating and extending Faraday's magnetic experiments,
