CONTENTS. PAGE THE MAGNETO-OPTIC PROPERTIES OF CRYSTALS AND THE RE- FORCE 193 SIXTH MEMOIR. ON THE RELATION OF DIAMAGNETIC POLARITY TO MAGNE- 225 243 250 281 283 PLATE II.-POLAR ANTITHESIS OF IRON AND BIS- THE ROYAL INSTITUTION ELECTRO-MAGNET. (See Frontispiece.) THE Electro-magnet represented in the Frontispiece is that generally used by Faraday in his researches on Diamagnetism. He employed a retort stand for suspension, covering the poles by a square glass shade, B C, to protect the suspended body from currents of air. The The magnet is formed from the link of a great chain-cable; its section is a distorted square, rounded off at the corners. magnet, coil inclusive, weighs 272 lbs. On the ends of the magnet stand two pieces of iron, P P, which are the movable poles. They represent those most commonly used by Faraday. Various other poles, however, with rounded, conical, and chisel ends, and some with perforations to allow a beam of light to pass through them, were employed from time to time. Right and left of the drawing, at R and L, are shown, in plan, the pole ends, with a little bar in its two characteristic positions, axial and equatorial, between them. To enable suspended conductors, such as copper cubes or spheres, to rotate in the magnetic field, with the axis of rotation parallel to the lines of force, I had the magnet supported by the pivot A, which permits its two arms to be placed, the one above the other, in a horizontal position. J. T. a FIRST MEMOIR. THE MAGNETO-OPTIC PROPERTIES OF CRYSTALS AND THE RELATION OF MAGNETISM AND DIAMAGNETISM TO MOLECULAR ARRANGEMENT. IN the year 1846 our views of magnetic action received, through the researches of Faraday, an extraordinary expansion. The experiments of Brugmans, Le Baillif, Seebeck, and Becquerel had already proved the power to be active beyond the limits usually assigned to it; but these experiments were isolated and limited in number. Faraday was the first to establish the broad fact, that there is no known body indifferent to magnetic influence when the latter is strongly developed. The nature of magnetic action was then found to be twofold, attractive and repulsive; thus dividing bodies into two great classes, which are respectively denominated magnetic and diamagnetic. The representative of the former class is iron, which, being brought before the single pole of a magnet, is attracted; the representative of the latter class is bismuth, which, being brought before the single pole of a magnet, is repelled.2 If a little bar of iron be hung freely between the two poles of a magnet, it will set its longest dimension in the 1 Published jointly with Professor Knoblauch in the Philosophical Magazine, July 1850, 2 Faraday afterwards suggested that the general term magnetism should include both the magnetism of iron and that of bismuth, which he respectively designated paramagnetism and diamagnetism. B |