Page:The American Cyclopædia (1879) Volume XI.djvu/15

 MAGNETISM TERRESTRIAL MAGNETISM 7 power. (See Comptes rendus, Paris, June, 1874.) Again, in the magnetization of iron, it is found that time is required to produce a full effect, as if it were necessary that inertia should be overcome ; and Mr. Grove has shown that, in rapidly changing the polarity of a bar by means of an alternating current of electri- city, the iron increases in temperature. The fact that a magnet heated to a white heat per- manently loses its magnetism is well known ; and in general the magnetism is diminished by any elevation of temperature. Dr. Maggie of Verona asserts that a circular plate of ho- mogeneous iron, when magnetized, conducts heat better in a direction perpendicular to the line joining the poles than in the direction of this line itself. It is also stated that iron strongly magnetized resists the action of the file in a greater degree than in its ordinary state. It was formerly supposed that mag- netism could be developed only in iron, nickel, and cobalt; but we now know from the re- searches of Faraday, that all bodies exhibit signs of an inductive influence, provided the magnetic power applied be sufficiently great. From the results of his experiments, Faraday was led to divide all bodies into two great classes : those like iron, nickel, and cobalt, which, on being suspended between the poles of an electro-magnet, assume an axial direc- tion, were denominated magnetic bodies, or paramagnetic; while those which arrange themselves at right angles to the magnetic meridian were denominated diamagnetic. (See DIAMAGNETISM.) The following series exhib- its some of the last results obtained by Fara- day on the magnetic and diamagnetic powers of bodies, in which the angle of torsion neces- sary to balance the force of a magnet expresses the power of the various substances, volume for volume, + representing the paramagnetic bodies, and the diamagnetic : proto-ammo- niate of copper, +134-23; oxygen, + 17'5; air, + 3'4; nitrogen, + 0'3; carbonic acid gas, 0-0; hydrogen, 0'1; glass, 18*2 pure zinc, 74'6; alcohol, 78-7 ; wax 86-73; nitric acid, 87-96 ; water, 96'6 sulphuric acid, 104-47; sulphur, 118 bismuth, 1967"6. Faraday discovered an other remarkable evidence of the action of magnetism on liquids and solids, as manifest in the effect produced on a polarized beam of light. Let a piece of gas pipe 18 inches long be closed at each end with a plate of tourma- line and filled with water. Let the axes of the tourmalines be placed transversely, so that the polarized beam of light which passes through the first may not be transmitted through the second. If while the apparatus is in this condition the iron be magnetized by a current of electricity passing through a long wire helix surrounding the tube, the beam of light will be partially transmitted by the sec- ond tourmaline. It is evident from this result that the magnetization of the iron has pro- duced an effect on the particles of the liquid, which has enabled them to react on the polar- ized beam of light and to produce as it were a twist in its plane of polarization. A simi- lar result will be produced if the liquid be con- tained in a tube of glass or any other sub- stance, and placed between the poles of a pow- erful magnet. To observe the effect however in this case, the poles of the magnet should be perforated for the transmission of the light. A similar effect is produced upon solid trans- parent bodies, and particularly upon heavy glass of the silicio-borate of lead. The phe- nomena of magnetism admit of being investi- gated quantitatively and mathematically with- out adopting any particular ideas as to the fundamental nature of this force ; the most complete investigations of this kind have been those of J. Clerk Maxwell (" Treatise on Elec- tricity and Magnetism," Oxford, 1873), who has been able thus to show the profound sig- nificance of Faraday's lines of force, and to make some progress in the reduction of this study to a dynamical science. Quite recently Bichat has published a very extended experi- mental investigation of this subject, and among other things has established the fact that the power of this magnetic influence diminishes as the temperature rises. Faraday also discover- ed the fact that crystallization exerts a con- siderable influence upon the direction of crys- tallized bodies placed between the poles of a powerful electro-magnet ; Plucker found that the axis of crystallization tended to assume the axial or equatorial direction ; and Tyndall and Knoblauch established the fact that if the mole- cules of any body are more condensed in one. direction than in any other, the magnetism will act along this direction with greatest intensity. If the substance is paramagnetic, the line of greatest condensation will assume an axial posi- tion ; if diamagnetic, the same line will come into a state of rest in the equator. This is shown by mixing carbonate of iron with gum into a stiff paste, a disk of which being com- pressed between the fingers, so as to give a greater density in one direction, and afterward suspended between the poles of a powerful elec- tro-magnet, will settle with its line of greatest condensation in the axial direction. If a simi- lar experiment be made with a compound of powdered bismuth and gum, the line of great- est condensation of this factitious substance will assume an equatorial position. Various attempts have been made to show a direct magnetizing influence in the solar beam to develop magnetism in soft iron needles, and it has even been asserted that the direct radia- tion from the moon has a powerful disturbing effect upon the needle of the mariner's com- pass ; but the most delicate experiments made by those best qualified for such investigations have failed to exhibit any result of this kind. MAGNETISM, Animal. See ANIMAL MAGNET- ISM. MAGNETISM, Terrestrial. Gilbert in 1600 was the first to announce the bold hypothesis that