Magnetism, the name given to the phenom-ena displayed by magnets. If a bar of slightly tempered steel be held vertically and struck several blows with a wooden mallet, it will acquire the property of attracting iron filings at its two extremities. The same property may be communicated from one bar of steel to any number of similar bars, by rubbing one half of the length of each of the latter with the end of the former which was toward the earth in the experiment above mentioned, and the remaining half with the other end of the same bar. In this process a remarkable fact becomes evident, namely, that the bar which is employed to impart the magnetic property loses none of its own power; on the contrary, if the process is properly performed, it will become stronger; and hence we deduce the conclusion, that in magnetization there is no transfer of any substance from one body to another, but the development of a latent principle. If a magnetized bar be suspended by a fibre of untwisted silk, in such a manner as to have perfect freedom of motion, it will assume a N. and S. direction; that is, it will exhibit the phenomena called polarity.

If to either end of a magnetized bar thus suspended a piece of soft iron be approached, attraction will be exhibited between them; when a similar bar is rolled in iron filings, the latter will be found to adhere in thick clusters at the two ends or poles, while none will attach themselves to the middle of the bar. If, instead of presenting to the suspended magnet pieces of soft iron, we bring near to its two ends in succession the two poles of another magnetized bar, repulsion as well as attraction will be exhibited; and by an attentive study of the phenomena we shall find that similarly magnetized ends repel, and dissimilarly magnetized ends attract each other. These forces act at great distances, through all interposed bodies, and like gravitation diminish in intensity with the square of the distance from each pole. If a number of bars of soft iron be placed near each other in the same straight line, and the N. end, for example, of a strongly magnetized steel bar be brought near one end of the series. each piece of iron will become magnetic and exhibit polarity.

The near end of the first magnet will be a S. pole, the far end a N. pole, and so on throughout the series, as follows:

S.. N. S. N. S. N S. N. S. N. S. N.

When the magnet is removed, the polarity of the iron bars ceases; and when the pole of the developing magnet is reversed, the polarity of the whole series is also reversed. The development of magnetism in this way is called induction, and by it we are enabled to explain many facts which would be otherwise perplexing. In accordance with this principle, we can assert that a magnet does not attract soft iron in its natural state, but that it first renders the metal magnetic, and then the attraction takes place between the dissimilar poles of two magnets. Again, when we sprinkle iron filings on a paper placed over a magnetic bar, they arrange themselves in beautiful curves radiating from each pole and joining near the equator of the bar. These lines result from the fact that each particle of iron becomes by induction a separate magnet, and attracts the adjacent filings, their arrangement in this case being the same as that of a series of small needles when under the influence of the two poles of a magnetic bar.

The induction takes place readily in soft iron, and disappears as soon as the inducing magnet is removed, but not so with hardened steel; though the effect is less powerful in this, the polarity is permanent. - The method of making steel magnets of great power, which we have found from long experience the sunplest and most efficient, is as follows: Procure say ten flat bars of good steel bout into the usual form of a horse shoe; let those be well hardened and fitted with their flat sides together so as to form a compound magnet.

Each of the members of this bundle may be magnetized separately to a small degree by supporting one of the legs on the lower end of a long rod of iron held nearly perpendicular in this latitude, and the other leg on the upper end of the same rod; or by rubbing one leg with the N. pole of a magnetized bar and the other with the S. pole. The several shoes, or bars, being in this way feebly magnetized, eight of them are joined together with their similar poles in contact, forming a compound magnet with which the remaining two bars are to be magnetized to a higher degree. Fortius purpose the latter are placed on a table on their flat sides, the X. pole of the one in contact with the S. pole of the other, so as to form a closed circuit; on any part of this circuit the compound horse shoe is placed perpendicular to the plane of the table, with its N. pole in the direction of the S. pole of the bar or shoe on which it rests. and then caused to slide in cither direction entirely around the circuit, care being taken to retain its perpendicularity. After having gone over the surface of the two shoes in this way several times, they are turned over without separating their ends, and the process is repeated on the side which was previously under.

By this method the two bars will receive a magnetic power nearly equal to the sum of the powers of the eight magnets in the bundle. Next these two bars are placed in the bundle, and two others are taken out and subjected to the same process. These in turn are put into the bundle, and two others are taken out and rubbed in the same way, until each pair of bars has been gone over two or three times in success-ion. By this method, with the most feeble beginning, the magnetism of the several may be developed to their full capacity, and a magnetic battery produced of great power. A compound horse shoe of this kind is the most convenient instrument for magnetizing straight bars of hardened steel for prac-Suppose, for example, we wish to magnetize bars, each 16 inches long, an inch wide, ami an eighth of an inch thick; these are placed on their flat sides in the form of a rectarngular parallelogram with their ends in cotact; the compound horse shoe is then placed perpendicularly on the middle of one of slid entirely around the paralleled times in succession; each bar is then turned over in its place so as to bring its side upward, and the process repeated care being taken to keep the horse shoe per-perpendicular to the plane of the parallelogram and its poles in the same relative positions to those of the bars.

By this method, if the compound horse shoe is sufficiently powerful the four ban can be magnetized to saturation in the course of a few minutes. If there are but two bars to be magnetized, the parallelogram is completed by joining the ends of these with two similar bars of soft iron, and the same process of rubbing performed as before - We have seen, in the article ElectroMagnetism, that the most powerful magnetic induction is produced in soft iron by transmitting around a bar of this metal a current of galvanism, and that temporary magnets of great power can be produced in this way. The same method affords the readiest means of strongly magnetizing steel bars. Whatever may be the nature of the change which takes place in iron at the moment of magnetization, we are certain that it pertains to the atoms or molecules of the body, and not to the assemblage of these as a whole. To be convinced of this, it is only necessary to magnetize a steel rod, for example a thick knitting needle, the polarity of which will be exhibited near its two ends, while no attraction will be manifested near the middle.

If however we break this into two pieces, we shall find each half is a perfect magnet; the separated ends which were previously joined together in the middle of the whole length will now exhibit polarity. If each of these pieces be again broken in two, we shall have four perfect magnets; and however frequent the division or small the parts into which the needle is divided, each part will still exhibit a NT. and S. pole. We may continue, at least in thought, this division, and we have no reason to doubt that however far it might be carried, the same result would be produced. We infer from this experiment that the reason why the middle of a bar exhibits no magnetism is not that none really exists there, but that it is neutralized by opposite polarities. We are also certain that magnetization is attended with at least a momentary motion of the atoms of the iron. This is proved by the fact that during the sudden magnetization of a bar of iron, by means of a current of electricity transmitted through a spiral conductor enclosing the bar, a sound is emitted; and if the bar be rapidly magnetized and demagnetized by an interruption of the current, a musical sound will be produced.

This fact was first noted by Dr. Page of the United States, and subsequently experimented upon by De la Rive, Becquerel, and others in Europe. The fact that a change takes place in the molecules is also rendered evident by an experiment of Mr. Joule of Manchester, England, in which he found that, although the whole capacity of the iron bar did not change on being magnetized, yet its dimensions varied, its length being increased and its width correspondingly diminished. That the magnetic force resides on or very near the surface of a magnet has been shown by Jamin, who finds that for every magnet there is a certain relation between the quantity of magnetism and the solid and superficial contents, such as to establish a limit beyond which a given bar cannot exert magnetic 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 electricity, the iron increases in temperature. The fact that a magnet heated to a white heat permanently 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 homogeneous 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 magnetism could be developed only in iron, nickel, and cobalt; but we now know from the researches 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 direction, 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 exhibits some of the last results obtained by Faraday on the magnetic and diamagnetic powers of bodies, in which the angle of torsion necessary 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 another 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 tourmaline 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 second tourmaline. It is evident from this result that the magnetization of the iron has produced an effect on the particles of the liquid, which has enabled them to react on the polarized beam of light and to produce as it were a twist in its plane of polarization. A similar result will be produced if the liquid be contained in a tube of glass or any other substance, and placed between the poles of a powerful 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 transparent bodies, and particularly upon heavy glass of the silicio-borate of lead.

The phenomena of magnetism admit of being investigated quantitatively and mathematically without 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 Electricity and Magnetism," Oxford, 1873), who has been able thus to show the profound significance 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 experimental 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 discovered the fact that crystallization exerts a considerable influence upon the direction of crystallized 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 molecules 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 position; 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 compressed between the fingers, so as to give a greater density in one direction, and afterward suspended between the poles of a powerful electro-magnet, will settle with its line of greatest condensation in the axial direction. If a similar experiment be made with a compound of powdered bismuth and gum, the line of greatest 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 radiation from the moon has a powerful disturbing effect upon the needle of the mariner's compass; but the most delicate experiments made by those best qualified for such investigations have failed to exhibit any result of this kind.