MAGNETISM.

Magnetic Iron—Knowledge of, by the Ancients—Artificial Magnets—Electro-Magnets—Electro-Magnetism—Magneto-Electricity—Theories of Magnetism—The Magnetic Power of soft Iron and Steel—Influence of Heat on Magnetism—Terrestrial Magnetism—Declination of the Compass-needle—Variation of the Earth’s Magnetism—Magnetic Poles—Hansteen’s Speculations—Monthly and Diurnal Variation—Dip and Intensity—Thermo-Magnetism—Aurora Borealis—Magnetic Storms—Magnetic conditions of Matter—Diamagnetism, &c.

Agreeably with the view now generally received, that magnetism and electricity are but modifications of one force, since they are found to stand to each other in the relation of cause and effect, the separation which is here adopted, of the consideration of their several phenomena, may appear inappropriate. The importance, however, of all that is connected with magnetism, and the very decided difference which is presented by true magnetic action, and that of frictional or chemical electricity, is so great that it has been thought advantageous to adopt the present arrangement in reviewing the influence of terrestrial magnetism with which science has made us acquainted.

From a very early period a peculiar attractive force has been observed in some specimens of iron ore. Masses of this kind were found in Magnesia, and from that locality we derive the name given to iron in its polar condition. This is confirmed by the following lines by Lucretius:—

Again we find Pliny employing the term magnetic, to express this singular power. It was known to the ancients that the magnetic power of iron, and the electric property of amber, were not of the same character, but they were both alike regarded as miraculous. The Chinese and Arabians seem to have known Magnetism at a period long before that at which Europeans became acquainted with either the natural loadstone or the artificial magnet. Previously to a.d. 121, the magnet is distinctly mentioned in a Chinese dictionary; and in a.d. 419 it is stated in another of their books that ships were steered south by it.[170]

The earliest popularly received account of its use in Europe is, that Vasco de Gama employed a compass in 1427, when that really adventurous navigator first explored the Indian seas. It is highly probable, however, that the knowledge of its important use was derived from some of the Oriental nations at a much earlier period.

We have some curious descriptions of the leading stone or loadstone, in the works of an Icelandic historian, who wrote in 1068. The mariner’s compass is described in a French poem of the date of 1181; and from TorfÆus’s History of Norway, it appears to have been known to the northern nations certainly in 1266.

We have not to deal with the history of magnetic discovery, but so far as it tells of the strange properties which magnets are found to possess, and the application of this knowledge to the elucidation of effects occurring in nature.

A brown stone, in no respect presenting anything by which it shall be distinguished from other rude stones around it, is found, upon close examination, to possess the power of drawing light particles of iron towards it. If this stone is placed upon a table, and iron filings are thrown lightly around it, we discover that these filings arrange themselves in symmetric curves, proceeding from some one point of the mass to some other; and upon examining into this, we shall find that the iron which has once clung to the one point will be rejected by the other. If this stone is freely suspended, we shall learn also that it always comes to rest in a certain position,—this position being determined by these points, and some attractive force residing in the earth itself. These points we call its poles; and it is now established that this rude stone is but a small representative of our planet. Both are magnetic: both are so in virtue of the circulation of currents of electricity, or of lines of magnetic force, as seen in the curves formed by the iron dust, and the north pole of the one attracts the south pole of the other, and the contrary. By a confusion of terms we speak of the north pole of a compass-needle, meaning that point which is always opposite to the north pole of the earth: the truth being that the pole of the compass-needle, which is so forcibly drawn to the north, is a point in a contrary state, or, as we may express it, really a south pole.

There is a power of a peculiar kind, differing from gravitation, or any other attracting or aggregating force with which we are acquainted, which exists permanently in the magnetic iron stones, and also in the earth. What is this power?

Magnetism may be produced in any bar of steel, either by rubbing it with a loadstone, or by placing it in a certain position in relation to the magnetic currents of the earth, and, by a blow or any other means, disturbing its molecular arrangement. This principle appears to involve the iron as with an atmosphere, and to interpenetrate it. By one magnet we may induce magnetism in any number of iron bars without its losing any of its original force. As we have observed of the electrical forces already considered, the magnet constantly presents two points in which there is a difference manifested by the circumstance that they are always drawn with considerable power towards the north or south poles of the earth. That this power is of the same character as the electricity which we have been considering, is now most satisfactorily proved. By involving a bar of soft iron which, being without any magnetic power, is incapable of sustaining even an ounce weight, with a coil of copper wire, through which a galvanic current is passing, the bar will receive, by induction from the current, an enormous accession of power, and will, so long as the current flows around it, sustain many hundred pounds weight, which, the moment the current is checked, fall away from it in obedience to the law of gravity. Thus the mere flow of this invisible agent around a mass of metal possessing no magneto-attractive power, at once imparts this life-like influence to it, and as long as the current is maintained, the iron is endowed with this surprising energy.

This discovery, which we owe to the genius of Oersted, and which has, indeed, given rise to a new science, electro-magnetism, may be regarded as one of the most important additions made to our knowledge.

Current electricity is magnetic; iron is not necessary to the production of magnetic phenomena, although by its presence we secure a greater amount of power. The copper wires which complete the circuit of a galvanic battery, will attract and hold up large quantities of iron filings, and the wires of the electric telegraph will do the same, while any signal is being conveyed along them. Again, all the phenomena common to galvanic electricity can be produced by merely disturbing the power permanently secured in the ordinary magnet. It was thought that magnets would become weakened by this constant disturbance of their magnetism; but, since its application to the purpose of manufacture, and magneto-electricity has been employed in electro-plating, it has been found that continued action for many years, during which enormous quantities of electricity have been thus given out and employed in producing chemical decomposition, has not, in the slightest degree, altered their powers. Thus a small bar of metal is shown to be capable of pouring out, for any number of years, the principle upon which the phenomena of magnetism depend.

There are, however, differences, and striking ones, between ordinary and magnetic electricity. In the magnet we have a power at rest, and in the electrical machine or galvanic battery, a power in motion. Ordinary electricity is stopped in its passage by a plate of glass, of resin, and many other substances; but magnetism passes these with freedom, and influences magnetic bodies placed on the other side. It would appear, though we cannot explain how, that magnetism is due to some lateral influence of the electric currents. A magnetic bar is placed over a copper wire, and it hangs steadily in the direction of its length; an electric current is passed along it, and the magnet is at once driven to place itself across the wire. Upon this experiment, in the main, AmpÈre founds his theory of terrestrial magnetism. He supposes electrical currents to be traversing our globe from east to west, and thus, that the needle takes its direction, not from the terrestrial action of any fixed magnetic poles, but from the repulsion of these currents, as is the case with the wire.

It has been found that wires, freely suspended, along which currents were passing in opposite directions, revolve about each other, or have an inclination to place themselves at right angles; thus exhibiting the same phenomenon as the magnet and the conducting wire. So far the hypothesis of AmpÈre leads us most satisfactorily. We see in the magnet one form of electricity, and in the machine or battery another. But why should not the electricity of the magnet, electricity at rest, exhibit the same powers as this force in motion?

Oersted, whose theory led him to the discovery of the fact of the magnetic power of an electric current, of the establishment indeed of the new science—Electro-Magnetism, regards the phenomena of a current passing a wire, and its action on a needle, as evidence of two fluids, positive and negative, traversing in opposite directions, and mutually attracting and repelling. He conceives that they pass the wires in a series of spirals; that in the magnet, by some peculiar property of the iron, this conflict of the currents is reduced to an equilibrium, and its power becomes manifested in its attractive force.[171] This does not, however, convey a clear idea to the mind.

It is curious that iron becomes magnetic in a superior degree to any other metal; that steel retains permanently any magnetism imparted to it; but that soft iron rapidly loses its magnetic power. This must be in virtue of some peculiar arrangement of the molecules, or some unknown physical condition of the atoms of the mass, by which a continued influence is retained by the steel, probably in a state of constant internal circulation. It has, however, been shown that soft iron, under certain circumstances, may be made to retain a large amount of magnetic force.[172]

If a horse-shoe shaped bar of soft iron is rendered magnetic by the circulation of an electric current around it, while its two ends are united by an armature of soft iron, so that it is capable of supporting many hundred pounds weight; and we then, by breaking the circuit, stop the current, taking care the armature is kept in contact, the iron will not lose its magnetic property, but will retain this power for many years. If the connecting piece of iron, the armature, is removed, the bar immediately loses all its magnetism, and will not support even the armature itself. This fact appears to confirm the idea that magnetism is due to the retention of electricity, and that steel possesses the property of equalizing the opposing forces, or of binding this principle to itself like an atmosphere.

The influence of heat on magnetism is so remarkable a proof of the dependence of this power upon molecular arrangement, that it must not escape our notice. To select but one of many experiments by Mr. Barlow, it was found that in a bar of malleable iron, in which, when cold, the magnetic effect was + 30° 0', all polarity ceased at a white heat, that it was scarcely appreciable at a red heat, but that at a blood-red heat it was equal to + 41° 0'.[173]

The more closely we examine the peculiarities of the magnetic power, and particularly as they are presented to us in its terrestrial action, the more surprising will its influence appear to be. We have discovered a natural cause which certainly exercises a very remarkable power over matter, and we have advanced so far in our investigations as to have learnt the secret of converting one form of force into another, or of giving to a principle, produced by one agency, a new character under new conditions; of changing, in fact, electricity into magnetism, and from magnetism again evolving many of the effects of electrical currents.

If a magnetic bar is freely suspended above the earth, it takes, in virtue of some terrestrial power, a given direction, which is an indication of the earth’s magnetic force. Whether this is the consequence of the currents of electricity, which AmpÈre supposes to circulate around the globe, from east to west, or the result of points of attraction in the earth itself, the phenomenon is equally wonderful. To whatever cause we may refer the visible effects, it appears certain that this earth is composed of particles in a magnetic state, the character varying with physical conditions, and that terrestrial magnetic force is the collective action of all the atoms of this planetary mass.[174]

The remarkable connexion which has been observed between the changes in the physical condition of the surface of the sun and terrestrial phenomena, must not escape our notice. Sir William Herschel thought he perceived a link connecting the dark spots on the sun’s face with the variations of the earth’s temperature. This has not, however, been confirmed by the observations which have been made since the time of Herschel. The careful examinations of the solar spots which have been made by Schwabe,[175] prove a well-defined order of progress in them. He has discovered that they move in cycles of ten years—from the smallest number visible in a given year, they regularly increase for five years, when they reach their maximum; they then as regularly decrease, and at the end of another five years they are at their maximum number. The magnetic observations which have been carried on by the British and other governments for some years, over every part of the world, have elicited the fact that the order of variation in the earth’s magnetic intensity is in cycles of ten years, and the law of increase and decrease which is found to prevail with the solar spots distinctly marks the variations of terrestrial magnetism. Few more interesting facts than this are within the range of our knowledge, proving as it does the direct dependence of terrestrial phenomena on solar force.

The constancy with which a magnetised needle points along a certain line which varies a little from the earth’s axial line, renders it one of the most important instruments to the practical and the scientific man. The wanderer of the ocean or of the desert is enabled, without fear of error, to pursue his path, and in unknown regions to determine the azimuth of objects. The miner or the surveyor finds in the magnetic compass the surest guide in his labours, and the experiment is for ever studying its indications.

has passed into a proverb among mankind, but the searching inquiry of modern observers has shown that the expression is correct only with certain limitations. There are two lines on the surface of the earth along which the needle points true north, or where the magnetic and the geographical north correspond. These are called lines of no variation, or, as they have also been designated, agonic lines, and one is found in the eastern and the other in the western hemisphere. The American line is singularly regular, passing in a south-east direction from the latitude 60° to the west of Hudson’s Bay, across the American lakes, till it reaches the South Atlantic ocean, and cuts the meridian of Greenwich in about 65° south latitude. The Asiatic line of no variation is very irregular, owing, without doubt, to local interferences; it begins below New Holland, in latitude 60° south, it bends westward across the Indian ocean, and from Bombay has an inflection eastward through China, and then northward across the sea of Japan, till it reaches the latitude of 71° north, when it descends again southward, with an immense semicircular bend, which terminates in the White Sea.

Hansteen has thought that there are two points in each hemisphere which may be regarded as stronger and weaker poles on opposite sides of the poles of revolution. These are called the magnetic poles of the earth, or by Hansteen magnetic points of convergence. These four points are considered to have a regular motion round the globe, the two northern ones from west to east, and the two southern ones from east to west. By the assistance of recorded observations, Hansteen has calculated the periods of these revolutions to be as follows:—

• The weakest north pole in 860 years.
• The strongest north pole in 1746 years.
• The weakest south pole in 1304 years.
• The strongest south pole in 4609 years.

There are some points of speculation on which Hansteen has ventured which have been smiled at as fanciful; but they may rather indicate an amount of knowledge in the Brahminical and Egyptian priesthood, beyond what we are usually disposed to allow them, and prove that their observations of nature had led them to an appreciation of some of the most remarkable harmonies of this mysterious creation.

The above terms are exceedingly near 864, 1246, 1728, 4320, and those numbers are equal to the mystic number of the Indians, Greeks, and Egyptians, 432 multiplied by 2, 3, 4, and 10. On these the ancients believed a certain combination of natural events to depend, and, according to Brahminical mythology, the duration of the world is divided into four periods, each of 432,000 years. Again, the sun’s mean distance from the earth is 216 radii of the sun, and the moon’s mean distance 216 radii of the moon, each the half of 432. Proceeding with this very curious examination, Hansteen says, 60 multiplied by 432 equals 15,920, the smallest number divisible at once by all the four periods of magnetic revolution, and hence the shortest time in which the four poles can complete a cycle, and return to their present state, and which coincides exactly with the period in which the precession of the equinoxes will amount to a complete circle, reckoning the precession at a degree in seventy-two years.[176]

When we consider the phenomena of terrestrial magnetism carefully, it appears to indicate the action of a power external to the earth itself, and, as Hansteen conceives, having its origin from the action of the sun, heating, illuminating, and producing a magnetic tension, in the same manner as it produces electrical excitation and actino-chemical action.

The movements of these magnetic poles have been the subject of extensive and most accurate observation in every quarter of the globe. In London, during 1657–1662, there was no magnetic variation; the agonic line passing through it. The variation steadily increased, until, in 1815, it amounted to 24° 15' 17", since which time it has been slowly diminishing. In addition to this great variation, we have a regular annual change dependent on the position of the sun, in reference to the equinoctial and solstitial points, which was discovered by Cassini, and investigated by Arago and others. Also a diurnal variation, which movement appears to commence early in the morning, moving eastward until half-past seven, a.m., when it begins to move westward until two, p.m., when it again returns to the east, and in the course of the night reaches the point from which it started twenty-four hours before.

We have also remarkable variations in what is termed the dip of the needle. It is well known that a piece of unmagnetized steel, if carefully suspended by its centre, will swing in a perfectly horizontal position, but, if we magnetize this bar, it will immediately be drawn downwards at one end. The force of the earth’s polarity, attracting the dissimilar pole, has caused it to dip.

There is, in the neighbourhood of the earth’s equator, and cutting it at four points, an irregular curve, called the magnetic equator, or aclinic line, where the needle balances itself horizontally. As we proceed from this line towards either pole the dip increases, until, at the north and south poles, the needle takes a vertical position. The intensity of the earth’s magnetism is also found to vary with the position, and to increase in a proportion which corresponds very closely with the dip. But the intensity is not a function of the dip, and the lines of equal intensity, isodynamic lines, are not parallel to those of equal dip. We have already remarked on the diurnal variation of the declination of the needle; we know, also, that there exists a regular monthly and daily change in the magnetic intensity. The greatest monthly change appears when the earth is in its perihelion and aphelion, in the months of December and June,—a maximum then occurs; and about the time of the equinoxes a minimum is detected.[177]

The daily variation of intensity is greatest in the summer, and least in the winter. The magnetism is generally found to be at a minimum when the sun is near the meridian; its intensity increasing until about six o’clock, when it again diminishes.[178]

What striking evidences all these well-ascertained facts give of the dependence of terrestrial magnetism on solar influence! and in further confirmation of this view, we find a very remarkable coincidence between the lines of equal temperature—the isothermal lines, and those of equal dip and magnetic intensity.

Sir David Brewster first pointed out that there were in the northern hemisphere two poles of maximum cold; these poles agree with the magnetic points of convergence; and the line of maximum heat, which does not run parallel to the earth’s equator, is nearly coincident with that of magnetic power. Since Seebeck has shown us that electrical and magneto-electrical phenomena can be produced by the action of heat upon metallic bars, we have, perhaps, approached towards some faint appreciation of the manner in which the solar calorific radiations may, acting on the surface of our planet, produce electrical and magnetic effects. If we suppose that the sun produces a disturbance of the earth’s electricity along any given line, in all directions at right angles to that line, we shall have magnetic polarity induced.[179] That such a disturbance is regularly produced every time the sun rises, has been sufficiently proved by many observers.

In 1750, Wargentin noticed that a very remarkable display of Aurora borealis was the cause of a peculiar disturbance of the magnetic needle; and Dr. Dalton[180] was the first to show that the luminous rays of the Aurora are always parallel to the dipping-needle, and that the Auroral arches cross the magnetic meridian at right angles. Hansteen and Arago have attended with particular care to these influences of the northern lights, and the results of their observations are:—

That as the crown of the Aurora quits the usual place, the dipping-needle moves several degrees forward:—

That the part of the sky where all the beams of the Aurora unite, is that to which a magnetic needle directs itself, when suspended by its centre of gravity:—

That the concentric circles, which show themselves previously to the luminous beams, rest upon two points of the horizon equally distant from the magnetic meridian; and that the most elevated points of each arch are exactly in this meridian.[181]

It does not appear that every Aurora disturbs the magnetic needle; as Captains Foster and Back both describe very splendid displays of the phenomenon, which did not appear to produce any tremor or deviation upon their instruments.[182]

Some sudden and violent movements have been from time to time observed to take place in suspended magnets; and since the establishment of magnetic observatories in almost every part of the globe, a very remarkable coincidence in the time of these agitations has been detected. They are frequently connected with the appearance of Aurora borealis; but this is not constantly the case. These disturbances have been called magnetic storms; and over the Asiatic and European continent, the islands of the Atlantic and the western hemisphere, they have been proved to be simultaneous.

From observations made at Petersburg by Kupffer, and deductions drawn from the observations obtained by the Magnetic Association, it appears probable that these storms arise from a sudden displacement in the magnetic lines of the earth’s surface; but the cause to which this may be due is still to be sought for.

In the brief and hasty sketch which has been given of the phenomena of terrestrial magnetism, enough has been stated to show the vast importance of this very remarkable power in the great operations of nature. We are gradually reducing the immense mass of recorded observations, and arriving at certain laws which are found to prevail. Still, the origin of the force, whether it is strictly electrical, whether it is the circulation of a magnetic fluid, or whether it is merely a peculiar excitation of some property of matter, are questions which are open for investigation.

In the beautiful Aurora borealis, with its trembling diffusive lights, and its many-coloured rays, we have what may be regarded as a natural exhibition of magnetism, and we appear to have within our grasp the explanation we desire. But we know not the secret of even these extraordinary meteorological displays. If we pass an electric spark from a machine through a long cylinder, exhausted of air as far as possible, we have a mimic representation of the Northern Lights—the same attenuation of brightness, almost dwindling into phosphorescence; and by the slightest change of temperature we may produce that play of colours which is sometimes so remarkably manifested in Aurora. Dr. Dalton considered Aurora borealis as a magnetic phenomenon, and that its beams are governed by the earth’s magnetism. We know that the arc of light produced between the poles of a powerful galvanic battery is readily deflected by a good magnet; and we have lately learned that every vapour obeys the magnetic force.[183] It is, therefore, yet a question for our consideration, does the earth’s magnetism produce the peculiar phenomena of Aurora by acting upon electricity in a state of glow? or have we evidence in this display of the circulation of the magnetic fluid around our globe, manifesting itself by its action on the ferruginous and other metallic matter, which Fusinieri has proved to exist in the upper regions of our atmosphere.[184] That magnetic radiations do exist, has been proved by Faraday, and that they form lines of force perpendicular to the earth’s surface, has been experimentally shown. Parallelograms of wire moved upon a central axis, and connected with a galvanometer, give at every revolution indication of an electric disturbance in all respects analogous to the production of a current by moving wires in front of a steel magnet.

The alteration in the properties of heat, when it passes from the radiant state into combination with matter, exhibits to us something like what we may suppose occurs in the conversion of magnetism into electricity or the contrary. We have a subtile agent, which evidently is for ever busy in producing the necessary conditions of change in this our earth: an element to which is due the development of many of the most active powers of nature; performing its part by blending with those principles which we have already examined; associating itself with every form of matter; and giving, as we shall presently see, in all probability, the first impulses to combination, and regulating the forms of aggregating particles.

As electricity has the power of altering the physical conditions of the more adherent states of matter, thus giving rise to variations of form and modes of combination, so gross matter appears to alter the character of this agency, and thus disposes it to the several modifications under which we have already detected its presence. We have mechanical electricity and chemical electricity, each performing its great work in nature; yet both manifesting conditions so dissimilar, that tedious research was necessary before they could be declared identical. Magnetic electricity is a third form; all its characteristics are unlike the others, and the office it appears to perform in the laboratory of creation is of a different order from that of the other states of electrical force. In the first two we have decomposing and recombining powers constantly manifested—in fact, their influences are always of a chemical character; but in the last it appears we have only a directive power. It was thought that evidence had been detected of a chemical influence in magnetism; it did appear that sometimes a retarding force was exerted, and often an accelerating one. This has been again denied, and we have arrayed in opposition to each other some of the first names among European experimentalists. The question is not yet to be regarded as settled; but, from long and tedious investigation, during which every old experiment has been repeated, and numerous new ones tried, we incline to the conclusion that chemical action is not directly affected by magnetic power. It is highly probable that magnetism may, by altering the structural arrangement of the surface, vary the rate of chemical action; but this requires confirmation.[185]

There is no substance to be found in nature existing independently of magnetic power. But it influences bodies in different ways: one set acting with relation to magnetism, like iron, and arranging themselves along the line of magnetic force,—these are called magnetic bodies; another set, of which bismuth may be taken as the representative, always placing themselves at right angles to this line,—these are called diamagnetic bodies.[186] This is strikingly shown by means of powerful electro-magnets; but the magnetism of the earth is sufficient, under proper care, to exhibit the phenomena.

Every substance in nature is in one or other of these conditions. The rocks, forming the crust of the earth, and the minerals which are discovered in them; the surface soil, which is by nature prepared as the fitting habitation of the vegetable world, and every tree, shrub, and herb which finds root therein, with their carbonaceous matter, in all its states of wood, leaf, flower, and fruit; the animal kingdom, from the lowest monad through the entire series up to man,—have, all of them, distinct magnetic or diamagnetic relations.

“It is a curious sight,” says Dr. Faraday, “to see a piece of wood or of beef, or an apple, or a bottle of water repelled by a magnet, or, taking the leaf of a tree, and hanging it up between the poles, to observe it take an equatorial position. Whether any similar effects occur in nature among the myriads of forms which, upon all parts of its surface, are surrounded by air, and are subject to the action of lines of magnetic force, is a question which can only be answered by future observation.”[187]

At present, the bodies which are known to exhibit decided ferro-magnetic properties are the following, which stand arranged in the order of their intensity:—

• Iron, Nickel, Cobalt, Manganese,
• Chromium, Cerium, Titanium,
• Palladium, Platinum, Osmium.

It is interesting to know that there are evidences that two bodies which, when separate, are not magnetic, as iron is, become so when combined. Copper and zinc are both of the diamagnetic class, but many kinds of brass are discovered to be magnetic.

The salts of the above metals are, to a greater or less extent, ferro-magnetic, but they may be rendered neutral by water, which is a diamagnetic body, being repelled by the magnet. It will be unnecessary, here, to enumerate the class of bodies which are diamagnetic; indeed, all not included in the preceding list may be considered as belonging to that class, with the exception of gases and vapours, which appear to exist, relatively to each other, sometimes in the one, and sometimes in the other condition.[188]

To endeavour to reduce our knowledge of these facts to some practical explanation, we must bear in mind that particular spaces around the north and south geographical poles of the earth, are regarded as circles to which all the magnetic lines of force converge. Under circumstances which should prevent any interference with what is called ferro-magnetic action, all bodies coming under that class would arrange themselves according to the laws which would regulate the disposition of an infinite number of magnets, free to move within the sphere of each other’s influence. The north and south pole of one magnetic body would attach itself to the south and north pole of another, until we had a line of magnets of any extent; the two ends being in opposite states, like the magnetic points of convergence of the earth.

Every body, not ferro-magnetic, places itself across such a line of magnetic force as we have conceived; and if the earth were made up of separate layers of ferro-magnetic and diamagnetic bodies, the result would be the formation of bands at right angles to each other. This is not the case, by reason of the intermingling of the two classes of substances. Out of the known chemical elements we find only about ten which are actively ferro-magnetic; the others combining with these give rise to either a weaker state, a neutral condition, or the balance of action is turned to the diamagnetic side. Sulphate of iron, for instance, is a magnetic salt; but in solution, water being diamagnetic, it loses its property. The yellow prussiate of potash dissolved in water is a diamagnetic body; but the red prussiate, which contains an atom less of potassium, is magnetic: but in the solid state they are both diamagnetic.[189]

From this it would appear that the chemical composition of a body regulated its relation to magnetism. The following facts will show, however, that the molecular structure is more particularly concerned in determining the molecular condition of substances.

M. PlÜcker, being desirous of finding the extent to which the direction of the fibres in organic bodies might influence their magnetic or diamagnetic properties, was led to inquire whether in crystals the direction of the optic axes, which itself depends upon the arrangement of the particles, might not also exercise some influence. The first submitted to the action of the electro-magnet a thin plate of tourmaline, such as is employed in experiments upon polarization, having its optic axis parallel to its longest length. It was very quickly perceived that the plate was magnetic, by the effect of the iron that it contains; but it was suspended successively in three ways,—first, so that its longest side was vertical, then as that the shortest side was vertical, and finally so that the plate itself was horizontal. In the first case it is directed between the two points of the conical curvatures of the poles like a magnetic body; but, in the other two cases, on the contrary, it took the direction assumed by diamagnetic bodies—that is to say, a direction such that its longest length was perpendicular to the line joining the poles. This direction indicated that the optical axis was repelled by the two poles, and that this repulsion outweighed the magnetic properties of the crystal.[190]

The relation of structure to physical phenomena of essentially different characters is remarkable. Savart, when making crystalline plates of quartz and carbonate of lime vibrate, succeeded in determining a relation between the acoustic figures that are produced in them, and the particular mode of the crystallization of the substance. He found that the direction of the optical axis is constantly connected with that of the principal forms of the acoustic figures.

Mitscherlich has remarked that crystals do not expand uniformly by heat, but that this dilatation is greater in one direction than in another; and that this difference is connected with their crystalline form. M. de SÉnarmont has shown that conductibility for heat, which is equal in all directions for the crystals of the regular system, acquires in others a maximum or a minimum value, according to directions parallel to the crystallographic axes; so that the isothermic surfaces, which are spheres in the former case, are, in the other, ellipsoids elongated or flattened in the same direction. The optical axes do not altogether coincide with the principal axes of conductibility for heat; but this appears to be due merely to slight differences in the rate of progression, or the refrangibility of the luminous and calorific rays.

Wiedemann, by employing a fine point through which he made electricity arrive upon a surface that he had powdered with licopodium or red lead, succeeded in determining, by means of the form assumed by this light powder, the conductibility of crystals in different directions.

On a surface of glass, the powder which disperses itself around the points, in consequence of electric repulsion, forms a circular figure traversed by radii. When a plate of gypsum is used instead of glass the figure is found to be elliptical, and the great axis of the ellipse forms a right angle with the principal crystallographic axis, which proves that the electricity distributes itself more easily in a direction perpendicular to the axis than in any other. M. Wiedemann comes to the conclusion that crystals which possess a better conductibility in the direction of the principal axis, all belong to the class of negative crystals: while those which have a better conductibility in the direction perpendicular to the axis are positive, which indicates that the direction of best conductibility for electricity is also that according to which light is propagated relatively with greater velocity.

Tyndale has shown, that if gutta percha which has been rendered fibrous in manufacture is cut so that the fibres are in the direction of this greatest length, or in a direction perpendicular to this greatest length, and placed under the influence of a magnet, they direct themselves equatorially. Ivory cut in the same direction manifests the same conditions, though both these substances are diamagnetic.

The fibrous structure, and the planes of cleavage, thus determine the magnetic condition of a substance. The special properties presented by crystals, in regard to the action exercised upon them by magnets, is due to a particular mode of grouping their particles. This is also the cause of unequal dilatability, and of unequal conductibility for heat and for electricity.

How curiously, therefore, does molecular structure determine the relation of a body to any of the forms of physical force!

We still search in the dark, and see but dimly the evidences; yet it becomes almost a certainty to us, that this stone of granite, with its curious arrangement of felspar, mica, and quartz, presents its peculiar condition in virtue of some law of magnetic force. The crystal, too, of quartz, which we break out of the mass, and which presents to us a beautifully regular figure, is, beyond a doubt, so formed, because the atoms of silica are each one impelled in obedience to one of these two conditions of magnetism to set themselves in a certain order to each other, which cannot be altered by human force without destruction.

All the laws which regulate the forms of crystals and amorphous bodies are, to the greatest degree, simple. In nature the end is ever attained by the easiest means; and the complexity of operation, which appears sometimes to the observer, is only so because he cannot see the spring by which the machine is moved.

The gaseous envelope, our atmosphere, is in a neutral state. Oxygen is strikingly magnetic in relation to hydrogen gas, whilst nitrogen is as singularly the contrary; and the same contrasts present themselves when these gases are examined in their relation to common air. Thus, oxygen being magnetic, and nitrogen the contrary, we have an equilibrium established, and the result is a compound neutral in its relations to all matter. All gases and vapours are found to be diamagnetic, but in different degrees.[191] This is shown by passing a stream of the gas, rendered visible by a little smoke, within the influence of a powerful magnet.

These bodies are, however, found relatively to each other,—or even to themselves, under different thermic conditions,—to change their states, and pass from the magnetic to the diamagnetic class. Heat has a very remarkable influence in altering these relations; and atmospheric air at one temperature is magnetic to the same fluid at another: thus, by thermic variations, attraction or repulsion may be alternately maintained. By this it must be understood that a stream of air, at a temperature elevated but a few degrees above that of an atmosphere of the same kind into which it is passing, is deflected in one way by a magnet; whereas, if the stream is colder than the bulk through which it flows, it is bent in another way by the same force. In this respect magnetism and diamagnetism show equally the influence of another physical force, heat; and we may safely refer many meteorological phenomena to similar alterations of condition in the atmosphere, relative to the magnetic relations of the aËrial currents.

That magnetism has a directive power is satisfactorily shown by the formation of crystals in the neighbourhood of the poles of powerful magnets. The common iron salt, the protosulphate, ordinarily crystallizes so that the crystals unite by their faces; but when crystallizing under magnetic influence, they have a tendency to arrange themselves with regard to each other so that the acute angle of one crystal unites with one of the faces of another crystal, near to, but never actually at, its obtuse angle. In addition to this, if a magnet of sufficient power is employed, the crystals arrange themselves in magnetic curves from one pole to the other, a larger crop of crystals being always formed at the north than at the south pole. Here we have evidence of an actual turning round of the crystal, in obedience to the directive force of the magnet; and we have the curious circumstance of a difference in some way, which is not clearly explained, between the two opposite poles. If, instead of an iron or a ferro-magnetic salt, we employ one which belongs to the other, or diamagnetic, class, we have a curious difference in the result. If into a glass dish, fixed on the poles of a strong electro-magnet, we pour a quantity of a solution of nitrate of silver, and place in the fluid, over the poles of the magnet, two globules of mercury (an arrangement by which that arborescent crystallization, called the Arbor DianÆ, is produced,) we have the long needle-shaped crystals of silver, arranging themselves in curves which would cut the ordinary magnetic lines at right angles.[192]

In the first example given we have an exhibition of magnetic force, while in the last we have a striking display of the diamagnetic power.

The large majority of natural formations appear to group themselves under the class of diamagnetics. These bodies are thought to possess poles of mutual repulsion among themselves, and which are equally repelled by the magnetic points of convergence. Confining our ideas to single particles in one condition or the other, we shall, to a certain extent, comprehend the manifold results which must arise from the exercise of these two modes of force. At present, our knowledge of the laws of magnetism is too limited to allow of our making any general deductions relative to the disposition of the molecules of matter; and the amount of observation which has been given to the great natural arrangements, is too confined to enable us to infer more than that it is probable many of the structural conditions of our planet are due to polarity.

Mountain ranges observe a singular uniformity of direction, and the cleavage planes of rock are evidently determined by some all-pervading power. Mineral bodies are not distributed in all rocks indiscriminately. The primary formations hold one class of metalliferous ores, and the more recent ones another. This is not to be regarded as in any way connected with their respective ages, but with some peculiar condition of the stone itself. The granite and slate rocks, at their junctions, present the required conditions for the deposit of copper ore, while we find the limestones have the characteristic physical state for accumulating lead ore. Again, on examining any mineral vein, it will be at once apparent that every particle of ore, and every crystal of quartz or limestone, is disposed in a direction which indicates the exercise of some powerful directive agency.[193]

It appears, from all the results hitherto obtained, that the magnetic and diamagnetic condition of bodies is equally due to some peculiar property of matter in relation to the other forms of electricity. We have not yet arrived at the connecting link, but it does not appear to be far distant.

We have already referred to the statement made by talented experimentalists, that magnetism has a powerful influence in either retarding or accelerating chemical combination. Beyond a doubt chemical action weakens the power of a magnet; but the disturbance which it occasions in soft iron, on the contrary, appears to tend to its receiving magnetism more readily, and retaining it more permanently. Further investigations are, however, required, before we can decide satisfactorily either of these problems, both of which bear very strongly upon the subject we have just been considering.

We have seen that heat and electricity act strangely on magnetic force, and that this statical power reacts upon them: and thus the question naturally arises, Do light and magnetism in any way act upon each other?

Morichini and Carpi on the continent, and Mrs. Somerville in England, have stated that small bars of steel can be rendered magnetic by exposing them to the influence of the violet rays of light. These results have been denied by others, but again repeated and apparently confirmed. In all probability, the rays to which the needles were exposed, being those in which the maximum actinic power is found, produced an actual chemical change; and then, if the position were favourable, it is quite evident that magnetism would be imparted. Indeed we have found this to be the case when the needles, exposed to solar radiations, were placed in the direction of the dip. The supposed magnetization of light by Faraday has already been mentioned. If the influence in one case is determined, it will render the other more probable.[194]

“In seeking for a cause,” writes Sir David Brewster, “which is capable of inducing magnetism on the ferruginous matter of our globe, whether we place it within the earth, or in its atmosphere, we are limited to the sun, to which all the magnetic phenomena have a distinct reference; but, whether it acts by its heat, or by its light, or by specific rays, or influences of a magnetic nature, must be left to future inquiry.”[195]

We have learnt that magnetism is not limited to ferruginous matter; we know that the ancient doctrine of the universality of the property is true. Kircher, in his strange work on Magnetism, published in the early part of the seventeenth century[196]—a curious exemplification of the most unwearying industry and careful experiment, combined with the influences of the credulity and superstitions of his age—attributes to this power nearly all the cosmical phenomena with which, in his time, men were acquainted. He curiously anticipates the use of the supposed virtue of magnetic traction in the curative art; and as the titles of his concluding chapters sufficiently show, he was a firm believer in animal magnetism.[197] But it is not with any reference to these that we refer to the work of Athanasii Kircheri, Societatis Jesu, Magnes, sive de Magnetiv Arte, but to show that two hundred years since, man was near a great truth; but the time of its development being not yet come, it was allowed to sleep for more than two centuries, and the shadow of night had covered it. In speaking of the vegetable world, and the remarkable processes by which the leaf, the flower, and the fruit are produced, this sage brings forward the fact of the diamagnetic character of the plant, which has been, within the last two years, re-discovered; and he refers the motions of the Sun-flower, the closing of the Convolvulus, and the directions of the spiral, formed by twining plants, to this particular influence.

This does not appear as a mere speculation, a random guess, but is the result of deductions from experiment and observation. Kircher doubtless leaped over a wide space to come to his conclusion; but the result is valuable in a twofold sense. In the first it shows us that, by neglecting a fact which is suggestive, we probably lose a truth of great general application; and secondly, it proves to us, that by stepping beyond the point to which inductive logic leads, and venturing on the wide sea of hypothesis, we are liable to sacrifice the true to the false, and thus to hinder the progress of human knowledge.

Magnetism, in one or other of its forms, is now proved to be universal, and to its power we are disposed to refer the structural conditions of all material bodies, both organic and inorganic. This view has scarcely yet been recognised by philosophers; but as we find a certain law of polarity prevailing through every atom of created matter, in whatever state it may be presented to our senses, it is evident that every particle must have a polar and directing influence upon the mass, and every coherent mass becomes thus only a larger and more powerful representative of the magnetic unit. Thus we see the speculation of Hansteen, that the sun is, to us, a magnetic centre, and that it is equally influenced by the remoter suns of the universe,[198] is supported by legitimate deductions from experiment.

The great difficulty is not, however, got rid of by this speculation; the cause by which the earth’s magnetism is induced is only removed further off.

The idea of a magnetic fluid is scarcely tenable; and the ferruginous nature of the Aurora borealis receives no proof from any investigation; indeed, we have procured evidence to show that iron is not at all necessary for the production of magnetic phenomena. The leaf of a tree, a flower, fruit, a piece of animal muscle, glass, paper, and a variety of similar substances, have the power of repelling the bar of iron which we call a magnet, and of placing it at right angles to the direction of the force exerted by them. This is a point which must be constantly borne in mind when we now consider the mysteries of magnetic phenomena.

Any two masses of matter act upon each other according to this law, and although by the power of cohesion the force may be brought to an equilibrium, or to its zero point, it is never lost, and may be readily and rapidly manifested by any of the means employed for electrical excitation.

Reasoning by analogy, the question fairly suggests itself: If two systems of inorganic atomic constitution are thus invested with a power of influencing each other through a distance, why may not two more highly developed organic systems equally, or to a greater extent, produce an influence in like manner? Upon such reasoning as this is founded the phenomenon known as Animal Magnetism. There is no denying the fact that one mass of blood, muscle, nerves, and bone, must, magnetically, influence another similar mass. This is, however, something totally different from that abnormal condition which is produced through some peculiar and, as yet, unexplained physiological influences.

With the mysterious operations of vital action, the forces which we have been considering have nothing whatever in common. The powers which are employed in the arrangements of matter are, notwithstanding their subtile character, of far too gross a nature to influence the psychological mysteries which present themselves to the observant mind. It cannot be denied that, by placing a person of even moderate nervous sensibility in a constrained position, and under an unnatural influence of the mind, as acquired by the disciples of Mesmer, a torpor affecting only certain senses is produced. The recognised and undoubted phenomena are in the highest degree curious—but in these the marvels of charlatanry and ignorance are not included;—and the explanation must be sought for by the physiologist among those hidden principles upon which depends all human sensation.[199]

Man, like a magician, stands upon a promontory, and surveying the great ocean of the physical forces which involve the material creation, and produce that infinite variety of phenomena which is unceasingly exhibited around him, he extends the wand of intelligence, and bids the “spirits of the vasty deep” obey his evocation.

The phenomena recur—the great processes of creation go on—the external manifestations of omnipotent power proceed—effects are again and again produced; but the current of force passes undulating onwards;—and to the proud bidding of the evocator there is no reply but the echo of his own vain voice, which is lost at last in the vast immensity of the unknown which lies beyond him.

We see how powerfully the physical forces, in their various modes of action, stir and animate this planetary mass; and amongst these the influence of magnetism appears as a great directing agent, though its origin is unknown to us.