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Magnesium.

grains of the metal into wire. The first great advance on Bussy's labors was in 1856, when Deville and Caron effected the reduction of the pure chloride of magnesium by mixing it with fused chloride of sodium in clay crucibles, using fluoride of calcium as a flux, and throwing in fragments of sodium; they thus obtained magnesium on a larger scale than any of their predecessors. The most important part of their investigations was the discovery of the volatility of the metal. All these were, however, mere laboratory experiments. In 1859 Bunsen of Heidelberg, and Roscoe (now of Manchester), published a memoir on the great importance of magnesium for photographic purposes, owing to the high refrangibility and the great actinic power of the light emitted by burning magnesium-wire. The study of this memoir led Mr. Sonstadt to consider whether, the magnesian salts being so abundant, the metal might not be obtained, on a comparatively large scale, at a moderate price. After a prolonged series of expensive experiments he succeeded, in 1862, in producing specimens of the metal varying from the size of a pin's head to that of a hen's egg. Although it burned freely enough, it was still wanting in ductility and malleability, in consequence of the presence of certain impurities; but by May, 1863, these difficulties were overcome by a process of purification by distillation; and by the close of that year he considered it safe to begin manufacturing. The magnesium metal company was consequently organized, and operations commenced at Manchester, where magnesium is now made on a considerable scale, as well as by an American magnesium company at Boston. One great advantage of Sonstadt's method is its simplicity; it can be accomplished by the hands of ordinary workmen ignorant of all chemical knowledge. The process of manufacture may be thus described: 1. An anhydrous chloride of magnesium is prepared by saturating lumps of rock-magnesia (carbonate of magnesia) with hydrochloric acid, and then evaporating the solution to dryness. 2. One part of metallic sodium cut in small pieces is placed in an iron crucible, and covered with five parts of the chloride. The crucible is covered, and heated to redness, when the chlorine leaves the magnesium and unites with the sodium, for which it has a stronger affinity. When the crucible has cooled, and its contents are removed en masse, and broken, the magnesium-in that state known as crude magnesium-is seen in nuggets of various sizes, varying from granules to masses as large as a hen's egg. 3. The distillation of the crude metal is effected in a crucible through which a tube ascends to within an inch of the lid. The tube opens at the bottom into an iron box, placed beneath the bars of the furnace, where, on the completion of the operation, magnesium is found in the form of a heap of drippings, which may be melted and cast into ingots or any desired form. The difficulty of obtaining a metal with so little ductility in the form of wire-the only form that was originally used for yielding light-had still to be overcome; and after various partially successful attempts to press small quantities into wire by Matthiessen and others, Mr. Mather of Salford devised a piece of machinery by which the metal is pressed into wire of various thickness. Mr. Mather also was the first who obtained the metal in ribbons, in which form, from the larger exposed surface, combustion takes place more completely. The apparatus for making the wire and ribbon is very ingenious. "The chief feature of it is a small hollow cylinder, adapted to receive a ram at one end, and covered at the other by an iron screen perforated with two or more holes opposite the chamber. This press, as the cylinder is called, is subjected to the action of gas from a blow-pipe, and the heat employed is only sufficient to soften the metal in the press. The pieces of magnesium are thrust into the chamber, the ram is placed in the mouth of the press, and a pressure of between two and three tons-obtained by hydraulic apparatus or by steam-forces the ram against the softened metal, and the latter oozes in continuous strings of wire through the perforations already named. To make ribbon, the wire thus obtained is passed between two hollow heated rollers, and is received in a flattened state upon a reel."-Richardson & Watts's Chemical Technology. To Mr. Mather is also due the credit of having constructed the first magnesium lamp, in which the end of the wire or ribbon is presented to the flame of a spirit-lamp. A concave reflector sent the light forward, and protected the eyes of the operator.

The first time that a photograph was taken by this light was at Manchester in the spring of 1864 by Mr. Brothers and Dr. Roscoe. That the magnesium light, in a more or less modified form, must prove of extreme value to photography cannot be called in question. Besides overcoming the obstacle of unsuitable weather for the employment of sunlight, it may be applied both for the exploration and the photography of various dim structures, underground regions, etc., such as the interior of the pyramids, of catacombs, natural caverns, etc., which could not otherwise be examined or photographed. Prof. Piazzi Smyth, the Scottish astronomer-royal, dating from the east tomb, great pyramid, Feb. 2, 1865, writes as follows: "With any number of wax candles which we have yet taken into either the king's chamber or the grand gallery, the impression left on the mind is merely seeing the candles and whatever is very close to them, so that you have small idea whether you are in a palace or a cottage; but burn a triple strand of magnesium wire, and in a moment you see the whole apartment, and appreciate the grandeur of its size and the beauty of its proportions." M. Madar is said to have taken a series of photographs of the catacombs of Paris; various artists are busy practicing on monuments in obscure recesses of continental churches; and in different parts of England caves of prehistoric interest either have been, or are about to be, photographed by this

Magnetism.

light. For portraiture, it is found to be less successful than was at first expected, owing to the intense light within a few feet of the sitter's eyes causing a contraction of the facial muscles.

Objectors to the application of such lights for the lighting of large buildings and thoroughfares maintain that, while light derived from oil or coal-gas, in which carbon constitutes the ignitible solid, possesses a power of diffusibility which renders objects not directly opposed to the course of the rays more or less distinctly visible, the electric, lime, and magnesium lights possess less of this diffusiveness; their rays being apparently projected with a force and velocity which interfere with the power of diffusion. An object placed in the direct course of the rays is splendidly illuminated, and the rays are projected to an immense distance; but the shadows cast by intervening objects are intensely black, and the rays seem to pass through the atmosphere without producing much effect, except upon those parts on which they directly fail.

We may now state some of the advantages which arise from the use of the magnesium light. Its color approaches very much nearer to daylight than that of the light from oils, candles, or coal-gas. As compared with the sun, its luminous intensity is , but its chemical intensity is, and this high actinic power makes it specially valuable for photographic purposes. Although it does not nearly equal the electric light as an illuminating agent, like it the magnesium light gives off no noxious vapors. But as it burns, white clouds of the vapor of magnesia are formed which would be more or less troublesome in private rooms. This objection is said to be to some extent removed, without diminishing the brilliancy of the light, by alloying with zinc; and at any rate, it would scarcely at all interfere with its use in large public buildings. Still less would it do so when the light is burned in the open air.

There is, however, not much hope of the magnesium light successfully competing with the electric light for the illumination of large buildings, streets, or even of ocean steamers. Recent trials with the electric light at the British museum and other places have now proved conclusively that wherever a great deal of light is required, gas is beaten out of the field on the score of economy. As respects the maintenance of an equal amount of light, gas is 20 times more costly, a difference which will speedily cover the original expense of the necessary electrical apparatus. The magnesium light, on the other hand, is much more costly than gas; and although the ores which could be used as a source of magnesium are very abundant, yet any probable cheapening of the process of extracting the metal from these is not likely to make the light a very economical one. Still, for any purpose where, for a comparatively brief time, a very intense light is required, magnesium wire or ribbon has about it almost the simplicity of a wax taper; nor are the lamps at all complex by which the metal may be burned for hours continuously.

Two kinds of magnesium lamps are made. In one of these kinds, wire or thin ribbon of the metal is coiled about a reel or bobbin. From this reel the ribbon is drawn by means of two small rollers and projected through a tube to the focus of a metallic reflector, where it passes through the flame of a spirit-lamp to insure its continuous combustion. These rollers are kept in motion either by an operator turning a small wheel, or in the more expensive forms by clock-work. In the other kind of lamp the magnesium is used in the form of dust, which is mixed with fine dry sand in the proportion of one of the former to two of the latter. This mixture is place in a funnel-shaped reservoir, and conducted, by means of a narrow tube provided with a stop-cock, to the flame of a spirit-lamp which serves to ignite and maintain the flame of the powdered magnesium. If nitrate of strontia be substituted for sand, a splendid red light is produced, and in this way, by using other chemicals, various colors can be obtained.

It was about the year 1864 that magnesium was first made on a commercial scale, and it is found that the demand for it, although not decreasing, is scarcely at all extending. It is almost wholly used for burning in photographic lamps, for flash lights, and for fire-works. It has been attempted to make magnesium useful for other purposes. Various alloys have been made with it and other metals, such as lead. tin, zinc, cadmium, and silver; but they are all brittle and liable to change. It is very doubtful, therefore, if any of these alloys will become useful in the arts, and the metal itself is scarcely likely to be available in the construction of objects of ornament or utility, since, when exposed to damp, it soon becomes coated with a film of hydrate of mag

nesium.

MAGNETIC CURES. It was held by physicians of old that the magnet exercised an important influence on the human body, or on the bodies of certain persons: this being shown in the alleviation of headache, toothache, cramp. etc. It has, however, been proved that the magnet as such has no influence on animal organisms, and that accordingly all cures professedly resting on such action have been due to delusion or deceit. But it is quite otherwise with magneto-electricity and galvanism. See ELECTRICITY, MEDICAL.

MAGNETIC IRON ORE. See LOADSTONE, ante.

MAGNETISM (said to be derived from the city Magnesia, where the loadstone was first discovered), is the power which the magnet has to attract iron. Under DIAMAGNETISM it is stated that every substance is more or less affected by the magnet, but as

Magnetism,"

iron is par excellence magnetic, the term is chiefly used with reference to it. Magnets are of two kinds, natural and artificial. Natural magnets consist of the ore of iron called magnetic, familiarly known as loadstone. Artificial magnets are, for the most part, straight or bent bars of tempered steel, which have been magnetized by the action of other magnets, or of the galvanic current.

Polarity of the Magnet.-The power of the magnet to attract iron is by no means equal throughout its length. If a small iron ball be suspended by a thread, and a magnet (fig. 1) be passed along in front of it from one end to the other, it is powerfully

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attracted at the ends, but not at all in the middle, the magnetic force increasing with the distance from the middle of the bar. The ends of the magnet where the attractive power is greatest are called its poles. By causing a magnetic needle moving horizontally to vibrate in front of the different parts of a magnet placed vertically, and counting the number of vibrations, the rate of increase of the magnetic intensity may be exactly found. Fig. 2. gives a graphic view of this increase. N S is the magnet; the lines n N. a a, etc., represent the magnetic intensities at the points N, a, etc., of the mag net; and the curve of magnetic intensity, Na Ma' n', is the line formed by the extremities of all the upright lines. It will be seen from the figure that the force of both halves, taking M as the dividing point, is disposed in exactly the same way, that for some distance on either side of the middle or neutral point there is an absence of force, and that its intensity increases with great rapidity towards the ends. The centers of gravity of the areas M N n and M S n are the poles of the magnet, which must therefore be situated near but not at the extremities.

A magnet has, then, two poles or centers of magnetic force, cach having an equal power of attracting iron. This is the only property, however, which they possess in common, for when the poles of one magnet are made to act on those of another, a striking dissimilarity is brought to light. To show this, let us suspend a magnet, N S, fig. 3, by a band of paper, M, hanging from a cocoon thread (a thread without torsion). When the magnet is left to itself, it takes up a fixed position, one end keeping north, and the other southi. The north pole cannot be made to stand as a south pole, and vice versa; for when the magnet is disturbed, both poles return to their original positions. Here, then, is a striking dissimilarity in the poles, by means of which we are enabled to distinguish them as north pole and south pole. When thus suspended, let us now try the effect of another magnet upon it, and we shall find that the pole of the suspended magnet that is attracted by one of the poles of the second magnet is repelled by the other, and vice versa; and where the one pole attracts, the other repels. If. now, the second magnet be hung like the first, it will be found that the pole which attracted the north pole of the first magnet is a south pole, and that the pole which repelled it is a north pole. We thus learn that each magnet has two poles, the one a north, the other a south pole, alike in their power of attracting soft iron, but differing in their action on the poles of another magnet, like poles repelling, and unlike poles attracting, each other.

M

FIG. 3.

It might be thought that, by dividing a magnet at its center, the two poles could be insulated, the one half containing all the north polar magnetism, and the other the south. When this is done, however, both halves become separate magnets, with two poles in each--the original north and south poles standing in the same relation to the other two poles cailed into existence by the separation. We can therefore never have one kind of magnetism without having it associated in the same magnet with the same amount of the opposite magnetism. It is this double manifestation of force which constitutes the polarity of the magnet.

The fact of the freely suspended magnet taking up a fixed position has led to the theory that the earth itself is a huge magnet, having its north and south magnetic poles in the neighborhood of the poles of the axis of rotation, and that the magnetic needle or suspended magnet turns to them as it does to those of a neighboring magnet. All the manifestations of terrestrial magnetism give decided confirmation of this theory. is on this view that the French call the north pole of the magnet the south pole (pôle

It

Magnetism.

austra), and the south the north pole (pole boréal); for if the earth be taken as the standard, its north magnetic pole must attract the south pole of other magnets, and vice versa. In England and Germany the north pole of a magnet is the one which, when freely suspended, points to the north, and no reference is made to its relation to the magnetism of the earth.

Form of Magnets.-Artificial magnets are either bar magnets or horse-shoe magnets. When powerful magnets are to be made, several thin bars are placed side by side, with their poles lying in the same way. They end in a piece of iron, to which they are bound by a brass screw or frame. Three or four of these may be put up into the bundle, and these again into bundles of three and four. Such a collection of magnets is called a magnetic magazine or battery. A magnet of this kind is more powerful than a solid one of the same weight and size, because thin bars can be inore strongly and regu larly magnetized than thick ones. Fig. 4 is a horse-shoe magnet magazine. The central lamina protrudes slightly beyond the other, and it is to it that the armature is attached, the whole action of the magnet being concentrated on the projection. A good form of magnet is a parallelopiped of magnetic iron ore, with pieces of soft iron, bound to its poles by a brass frame encircling the whole. The lower ends of the soft iron bars act as the poles, and support the armature. The magnetic needle is a small magnet nicely balanced on a fine point. See COMPASS.

Magnetic Induction.-When a short bar of soft iron is suspended from one end of a magnet it becomes for the time powerfully magnetic. It assumes a north and south pole, like a regular magnet, as may be seen by using a small magnetic needle; and if its lower end be dipped into iron filings, it attracts them as a magnet would do. When it is taken away from the magnet the filings fall off, and all trace of magnetism disappears. It need not be in actual contact to show magnetic properties; when it is simply brought near, the same thing is seen, though to a less extent. If the inducing magnet be strong enough, the induced magnet, when in contact, can induce a bar like itself, placed at its extremity, to became a magnet; and this second induced magnet may transmit the magnetism to a third, and so on, the action being, however, weaker each time. If a steel bar be used for this experiment, a singular difference is observed in its action; it is only after some time that it begins to exhibit magnetic properties, and, when exhibited, they are feebler than in the soft iron bar. When the steel bar is removed, it does not part instantly with its magnetism, as the soft iron bar, but retains it permanently. Steel, therefore, has a force which, in the first instance, resists the assumption of magnetism; and, when assumed, resists its withdrawal. This is called the coercitive force. The harder the temper of the steel, the more is the coercitive force developed in it. it is this force also, in the loadstone, which enables it to retain its magnetism.

FIG. 4.

H

Magnetization.-By single touch (Fr. simple touche, Ger. einfacher strich): The steel bar to be magnetized is laid on a table, and the pole of a powerful magnet is rubbed a few times along its length, always in the same direction. If the magnetizing pole be north, the end of the bar it first touches each time becomes also north, and the one where it is lifted south. The same thing may be done by putting, say, the north magnetizing pole first on the middle of the bar, then giving it a few passes from the middle to the end, returning always in an arch from the end to the middle. After doing the same to the other half with the south pole, the magnetization is complete. The first end rubbed becomes the south, and the other the north pole of the new mag. net. By divided touch (Fr. touche séparée, Ger. getrennter strich): The bar to be magnetized is placed on a piece of wood with its ends abutting on the extremities of two powerful magnets. Two rubbing magnets are placed with their poles together on the middle, inclined at an angle rather less than 30° with it. They are then simultaneously moved away from each other to the ends, and brought back in an arch again to the middle. After this is repeated a few times, the bar is fully magnetized. This method communicates a very regular magnetism, and is employed for magnetic needles, or where accuracy is needed. The magnetization by double touch is of less practical importance, and need not here be described. It communicates a powerful but sometimes irregular magnetism, giving rise to consecutive poles-that is, to more poles than two in the magnet.

For horse-shoe magnets, Hoffer's method is generally followed. The inducing magnet is placed vertically on the magnet to be formed, and moved from the ends to the bend, or in the opposite way, and brought round again, in an arch, to the starting-point. A soft iron armature is placed at the poles of the induced magnet. That the operation may succeed well, it is necessary for both magnets to be of the same width. The same method may also be followed for magnetizing bars. The bars with the armatures are placed so as to form a rectangle; and the horseshoe-magnet is made to glide along both in the way just described.

Magnetization by the Earth.-The inductive action of terrestrial magnetism is a striking proof of the truth of the theory already referred to, that the earth itself is a magWhen a steel rod is held in a position parallel to the dipping-needle (q. v.), it

net.

Magnetism.

becomes in the course of time permanently magnetic. This result is reached sooner when the bar is rubbed with a piece of soft iron. A bar of soft iron held in the same position is more powerfully but only temporarily affected, and when reversed, the poles are not reversed with the bar, but remain as before. If when so held it receive at its end a few sharp blows of a hammer, the magnetism is rendered permanent, and now the poles are reversed when the bar is reversed. The torsion caused by the blows of the hammer appears to communicate to the bar a coercitive force. We may understand from this how the tools in work-shops are generally magnetic. Whenever large masses of iron are stationary for any length of time they are sure to give evidence of magnetization, and it is to the inductive action of the earth's poles acting through ages that the magnetism of the loadstone is to be attributed.

Preservation and Power of Magnets.—Magnets, when freshly magnetized, are sometimes more powerful than they afterwards become. In that case they gradually fall off in strength till they reach a point at which their strength remains constant. This is called the point of saturation. If a magnet has not been raised to this point, it will lose nothing after magnetization. We may ascertain whether a magnet is at saturation by magnetizing it with a more powerful magnet, and seeing whether it retains more magnetism than before. The saturation point depends on the coercitive force of the magnet, and not on the power of the magnet with which it is rubbed. When a magnet is above saturation, it is soon reduced to it by repeatedly drawing away the armature from it. After reaching this point, magnets will keep the same strength for years together if not subjected to rough usage. It is favorable for the preservation of magnets that they be provided with an armature or keeper. For further information, see article ARMATURE. The power of a horse-shoe magnet is usually tested by the weight its armature can bear without breaking away from the magnet. Häcker gives the following formula for this weight: Wa m2; Wis the charge expressed in pounds; a, a constant to be ascertained for a particular quality of steel; and m is the weight in pounds of the magnet. He found, in the magnets that he constructed, a to be 12.6. According to this value, a magnet weighing 2 oz. sustains a weight of 3 lbs. 2 oz., or 25 times its own weight; whereas a magnet of 100 lbs. sustains only 271 lbs., or rather less than 3 times its own weight. Small magnets, therefore, are stronger for their size than large ones. The reason of this may be thus explained: Two magnets of the same size and power, acting separately, support twice the weight that one of them does; but if the two be joined, so as to form one magnet, they do not sustain the double, for the two magnets being in close proxim-, ity, act inductively on each other, and so lessen the conjoint power. Similarly, several magnets made up into a battery have not a force proportionate to their number. Large magnets in the same way may be considered as made up of several laminæ, interfering mutually with each other, and rendering the action of the whole very much less than the sum of the powers of each. The best method of ascertaining the strength of bar magnets is to cause a magnetic needle to oscillate at a given distance from one of their poles, the axis of the needle and the pole of the magnet being in the magnetic meridian. These oscillations observe the law of pendulum motion, so that the force tending to bring the needle to rest is proportionate to the square of the number of oscillations in a stated time.

Action of Magnets on each other.-Coulomb discovered, by the oscillation of the magnetic needle in the presence of magnets in the way just described, that when magnets are so placed that two adjoining poles may act on each other without the interference of the opposite poles, that is, when the magnets are large compared with the distance between their centers, their attractive or repulsive force varies inversely as the square of the distance. Gauss proved from this theoretically, and exhibited experimentally, that when the distance between the centers of two magnets is large compared with the size of the magnets, that is, when the action of both poles comes into play, their action on each other varies inversely as the cube of the distance.

Effect of Heat on Magnets.-When a magnet is heated to redness it loses permanently every trace of magnetism; iron, also, at a red heat, ceases to be attracted by the magnet. At temperatures below red heat the magnet parts with some of its power, the loss increasing with the temperature. The temperatures at which other substances affected by the magnet lose their magnetism differ from that of iron. Cobalt remains magnetic at the highest temperatures, and nickel loses this property at 662° F.

Ampere's Theory of Magnetism.-This theory forms the link between magnetism and galvanic electricity, and gives a simple explanation of the phenomena of electro-magnetism and magneto-electricity. We shall therefore preface the short discussion of these two subjects by a reference to it. Ampere considers that every particle of a magnet has closed currents circulating about it in the same direction. A section of a magnet according to this theory is shown in fig. 5. All the separate currents in the various particles may, however, be considered to be equivalent to one strong current circulating round the whole (fig. 6). We are to look upon a magnet, then, as a system, so to speak, of rings or rectangles, placed side by side, so as to form a cylinder or prism, in each of which a current in the same direction is circulating. Before magnetization the currents run in different directions, so that their effect as a system is lost, and the effect of induction is to bring them to run in the same direction. The perfection of magnetization is to render the various currents parallel to each other. Soft iron, in consequence

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