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ring, is heated at one of the junctions by a lamp, a current of electricity flows through the circuit from the antimony to the bismuth; and such thermo-electric currents produce all the electro-magnetic effects. A compass needle, placed either within or without the circuit, and at a small distance from it, is deflected from its natural position, in a direction corresponding to the way in which the electricity is flowing. If such a ring be suspended so as to move easily in any direction, it will obey the action of a magnet brought near it, and may even be made to revolve. According to the researches of M. Seebeck, the same substance, unequally heated, exhibits electrical currents; and M. Nobili observed, that in all metals, except zinc, iron, and antimony, the electricity flows from the hot part towards that which is cold. That philosopher attributes terrestrial magnetism to a difference in the action of heat on the various substances of which the crust of the earth is composed; and, in confirmation of his views, he has produced electrical currents by the contact of two pieces of moist clay, of which one was hotter than the other.

M. Becquerel constructed a thermo-electric battery of one kind of metal, by which he has determined the relation between the heat employed and the intensity of the resulting electricity. He found that, in most metals, the intensity of the current increases with the heat to a certain limit, but that this law extends much farther in metals that are difficult to fuse, and which do not rust. The experiments of Professor Cumming show that the mutual action of a magnet and a thermo-electric current is subject to the same laws as those of magnets and galvanic currents; consequently all the phenomena of repulsion, attraction, and rotation may be exhibited by a thermo-electric current. M. Botto, of Turin, has decomposed water and some solutions by thermo-electricity; and the Cav. Antinori of Florence succeeded in obtaining a brilliant spark with the aid of an electro-dynamic coil.

The principle of thermo-electricity has been employed by MM. Nobili and Melloni for measuring extremely minute quantities of heat in their experiments on the instantaneous transmission of radiant heat. The thermo-multiplier, which they constructed for that purpose, consists of a series of alternate bars, or rather fine wires of bismuth and antimony, placed side by side, and the extremities alternately soldered together. When heat is applied

to one end of this apparatus, the other remaining at its natural temperature, currents of electricity flow through each pair of bars, which are conveyed by wires to a delicate galvanometer, the needle of which points out the intensity of the electricity conveyed, and consequently that of the heat employed. This instrument is so delicate that the comparative warmth of different insects has been ascertained by means of it.

The conservation of force is strictly maintained throughout the whole science and different forms of electricity. In static electricity the positive and negative forces exactly balance one another; they are always simultaneous, and related often by curved lines of force; there is no defect or surplus, and the existence of one kind without the other is utterly impossible—it is absolutely a dual force. The very same may be said of electric currents, whether produced by the Voltaic battery or in any other way-the current in one part of the circuit is absolutely the same in amount and dual character as the other; and in the insulated Voltaic battery, where the sustaining power is internal, not the slightest development of the forces of either of these can occur till the circuit is completed or induction allowed at the extremities; for if when there is no circuit the induction be prevented, not merely no current, but no quantity of electricity at the poles ready to produce a current, can be evolved in the slightest degree.*

* Faraday.

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SECTION XXXIV.

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Magnetism a Dual Power-Antithetic Character of Paramagnetism and Diamagnetism The Earth Paramagnetic - Properties of Paramagnetic Bodies Polarity -Currents Induction Lines of Magnetic Forceof Electricity induced by them-Proved to be Closed Curves - Analogy and Identity of Electricity and Magnetism Terrestrial Magnetism Mean Values of the Three Magnetic Elements - Their Variations in Double Progression proved to consist of Two Superposed Variations Discovery of the Periodicity of the Magnetic Storms - The Decennial Period of the Magnetic Elements the same with that of the Solar Spots Magnetism of the Atmosphere - Diamagnetism - Action of ElectroMagnetism on Paramagnetic, Diamagnetic Bodies, and on Copper, very different Proof of Diamagnetic Polarity and Induction Magnecrystallic Action - Effects of Compression, Heat, and Cleavage on Magnetic Bodies - Mutual Dependence of Light, Heat, Electricity, &c. &c. The Conservation of Force and the Permanency of Matter Primary Laws of Nature-Definition of Gravity not according to that Law Gravity only the Residual Force of a Universal Power-Magnetism of the Ethereal Medium.

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MAGNETISM may be regarded as a new science in consequence of the profound researches and admirable discoveries of Dr. Faraday. Since the magnetism of matter is only known by the action of a magnet or of electricity upon it, by using an extremely energetic magnet or electro-magnet he has proved that all known substances, whether solid, liquid, or aëriform, are more or less magnetic, but that the magnetism is very different in different substances. For example, if a bar of iron be freely suspended between the poles of a very powerful magnet or electromagnet, it will be attracted by both poles, and will set or rest in the direction of a straight line joining them; but if a similar bar of bismuth be freely suspended in the same manner, it will rest in a direction at right angles to that which the iron bar assumed. Thus the direction in which the iron sets is axial or in the line of force, while that which the bismuth assumes is equatorial or perpendicular to the line of force. Substances that are magnetic after the manner of iron are said to be paramagnetic, those that are magnetic after the manner of bismuth are diamagnetic. As far as we know, all matter comes under one or

other of these laws. Many bodies are paramagnetic besides iron, as the loadstone, which consists of the peroxide and protoxide of iron mixed with small portions of silica and alumina ; also some of the gems and metals, as cobalt, nickel, &c. A substance is often paramagnetic if it contains only the 130,000th part of its weight of iron; but by far the greater number are diamagnetic, as all animal and vegetable matter, acids, oils, sugar, starch, bread, &c., and all the gases except oxygen, which is highly paramagnetic; and its force increases with its density: but notwithstanding the predominance of diamagnetic matter at the surface, the terrestrial globe is paramagnetic-in fact it is a powerful magnet.

Besides the substances which are paramagnetic naturally, that property may be imparted by a variety of methods, as by friction with magnets or even juxtaposition with them; and a bar of hard steel held at the angle of the dip will become a magnet on receiving a few strokes with a hammer on its upper end.

Polarity is one of the most distinguishing characters of magnetism: it is the property which a magnet possesses when freely suspended of resting spontaneously in the magnetic meridian, or nearly north and south, and always returning to that position when disturbed in consequence of the mean magnetic attraction of the earth; yet the magnet has no tendency to move to the north or south even when floating on water, because the same pole that attracts one end repels the other. Both poles of a magnet attract iron, which in return attracts either pole of the magnet with an equal and contrary force. The action of a magnet on unmagnetised iron is confined to attraction, whereas the reciprocal agency of magnets is characterised by a repulsive as well as by an attractive force; for a north pole repels a north pole, and a south pole repels a south pole; but a north and south pole mutually attract one another—which proves that paramagnetism is a dual power in which the conservation of force is perfectly maintained, for the force of attraction is exactly equal to the force of repulsion. One kind of polarity cannot exist without the other: they are absolutely simultaneous, dependent, and of equal intensity.

Induction is the power which a magnet possesses of exciting temporary or permanent paramagnetism in such bodies in its vicinity as are capable of receiving it. By this property the mere approach of a magnet renders iron and steel paramagnetic,

the more powerfully the less the distance, but the induced force is always exactly equal to the force which produces it. When the north end of a magnet is brought near to, and in the line with, an unmagnetised iron bar, the bar acquires all the properties of a perfect magnet; the end next the north pole of the magnet becomes a south pole, while the remote end becomes a north pole. Exactly the reverse takes place when the south end is presented to the bar, so that each pole of a magnet induces the opposite polarity in the adjacent end of the bar, and the same polarity in the remote extremity; consequently the nearest extremity of the bar is attracted, and the farther repelled; but as the action is greater on the adjacent than on the distant part, the resulting force is that of attraction. By induction the iron bar not only acquires polarity, but the power of inducing paramagnetism in a third body; and although all these properties vanish from the iron as soon as the magnet is removed, a lasting increase of intensity is generally imparted to the magnet itself by the reaction of the temporary paramagnetism of the iron. Iron acquires the inductive force more rapidly than steel, yet it loses it as quickly on the removal of the magnet, whereas the steel is impressed with a lasting polarity.

A certain time is requisite for induction, and it may be accelerated by anything that excites a vibratory motion in the particles of the steel; such as the smart stroke of a hammer, or heat succeeded by sudden cold. A steel bar may be converted into a magnet by the transmission of an electric discharge through it; and as its efficacy is the same in whatever direction the electricity passes, the effect arises from its mechanical operation exciting a vibration among the particles of the steel. It has been observed that the particles of iron easily resume their neutral state after induction, while those of steel resist the restoration of equilibrium, or a return to the neutral state: it is therefore evident that any cause which removes or diminishes the resistance of the particles will tend to destroy the paramagnetism of the steel; consequently the same mechanical means which develop the power will also destroy it. On that account a steel bar may lose its paramagnetism by any mechanical concussion, such as by falling on a hard substance, a blow with a hammer, and heating to redness, which makes the steel soft. The circumstances which determine whether it shall gain or lose are its position with

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