The atmosphere which surrounds the earth to the height of about fifty miles with sensible density, consists of three and a half parts by weight of nitrogen gas and one part of oxygen, uniformly mixed. The nitrogen is neutral whether dense or rare, hot or cold, while the oxygen is highly paramagnetic; but it loses a great part of its force when rarefied by heat; consequently the magnetic force of the atmosphere must increase from the equator to the poles of maximum cold; it must vary summer and winter, night and day. Its effect upon terrestrial magnetism is unknown; but it can hardly be without some influence. M. E. Becquerel observes—“If we reflect that the earth is encompassed by a mass of air equivalent in weight to a layer of mercury of 30 inches, we may inquire whether such a mass of magnetic gas, continually agitated, and submitted to the regular and irregular variations of pressure and temperature, does not intervene in some of the phenomena dependent upon terrestrial magnetism. If we calculate, in fact, what is the magnetic force of this fluid mass, we find that it is equivalent to an immense plate of iron, of a thickness little more than 1⁄250 of an inch, which covers the whole surface of the globe.” Both the conducting power of the air and its density are increased by cold; and as the sum of the magnetic forces which issue from the earth on one side of the line of no dip is equal to their sum on the other side, the intensity and concentration in our winter are coincident with a diffusion and feebleness in the opposite hemisphere, so that the line of no dip will move annually from north to south and back again. The same holds with regard to day and night. Thus the law of the conservation of force is rigorously maintained; and it is equally so in the effect of the atmosphere on the magnetic lines of force, which refracts them as they pass through it, in one direction in summer, and in the opposite direction in winter—in one direction in the enlightened hemisphere, in the other in that which is dark. The whole of the magnetic lines about the earth are held by their mutual tension in one connected, sensitive system, which feels in every part, even to the antipodes, a change in any particular place.
It may be mentioned as a well-known fact, that apparent anomalies have been found in the diurnal variation of the declination in the high magnetic latitudes of the northern hemisphere when compared with their great regularity in other parts of the same hemisphere, and that the magnetic storms are of much greater magnitude there than in lower latitudes. Moreover, although Captain Maguire’s observations at Cape Barrow, in the North Polar Ocean, show that the annual and diurnal variations of the casual disturbances or magnetic storms, as well as those of the decennial period, are maintained, yet it appears that at certain hours of the day the disturbance in the declination may be easterly at Point Barrow, and westerly at the Magnetic Observatory at Toronto, in Upper Canada, and vice versâ: in fact, the magnetic storms are simultaneous at these two stations, but in opposite directions—a circumstance not yet accounted for, and may possibly be due to the increased magnetism of the air in these cold regions. The heat of the sun has no effect upon terrestrial magnetism unless possibly by its indirect action on the oxygen of the atmosphere; but hitherto it has been imperceptible. It is hardly possible that the aurora can be independent of the magnetic character of the air, since it occurs in the high latitudes, where the atmospheric magnetism is most powerful. Captain Maguire remarked that it frequently appeared at Point Barrow when the magnetic storms were at a maximum.
We are totally ignorant of the cause of terrestrial magnetism, though the powerful influence of the solar spots renders it highly probable that it will ultimately be found to originate in the sun himself. Mr. Barlow’s theory of electric currents revolving round the globe is borne out by Mr. Fox’s observations in the Cornish mines, which show that electro-magnetism is extremely active in metallic veins; that not only the nature of the metalliferous deposits must have been determined by their relative electrical conditions, but that the direction of the metallic veins must have been influenced by the direction of the magnetic meridians, and in fact almost all the metallic deposits in the world tend from east to west, or from north-east to south-west. However, these currents of electricity may be regarded as magnetic lines of force, and are more likely to be the effect than the cause of terrestrial magnetism. They are found to have a powerful inductive effect on the Atlantic telegraph, disturbing the needles and galvanometers at each end of the line to a considerable degree, and on the night of the 6th of September, 1858, a magnetic storm passed over the cable, which violently agitated the reflecting galvanometer in connection with the telegraphic wires.
We are equally ignorant of the cause of the secular magnetic variations, but we have no reason to believe that the earth is alone magnetic; on the contrary, the planets are probably magnets, and we know that the sun and moon are magnetic; hence, as the magnetic, like the gravitating force, is transmitted through the ethereal medium, the induction of the sun, moon, and planets, in all their secular and periodic changes, may cause perpetual variations in terrestrial magnetism, and it may not be beyond the delicacy of modern observation to ascertain whether a planet, when nearest to the earth, has any sensible magnetism.
Diamagnetism is also a dual power, but in complete antithesis to paramagnetism under the same circumstances. Dr. Faraday first discovered this property in heavy glass, or silico-borate of lead, a piece of which was repelled by the pole of a powerful electro-magnet, and an elongated prism of the same heavy glass, when freely suspended between the poles, set equatorially. He then found that so great a number of substances followed the same law, that it established the very remarkable fact of a hitherto unknown force having acted upon the substances submitted to its influence, a discovery which he subsequently confirmed by many experiments, all of which proved the antithesis between the two modes of magnetic action. He also discovered that magnetic bodies differ exceedingly in their magnetic power: of paramagnetic bodies iron is the most powerful; then follow nickel, cobalt, and a long gradation down to osmium and a vacuum. The body that seems to have the lowest diamagnetic power is arsenic, and the series ascends to heavy glass, antimony, phosphorus, and bismuth; so iron and bismuth are the most powerful in their respective classes, and both have a small conducting power for electricity. It may be presumed that many remarkable instances of diamagnetism are to be met with in nature; among others, Dr. Faraday has suggested the idea that Saturn’s ring, from its position, may be diamagnetic with regard to the planet.
With very powerful magnets or electro-magnets, which are absolutely necessary for all these experiments, it is found that no simple substance is neutral, but that such may be compounded by mixing in due proportion a diamagnetic and paramagnetic liquid, as water and protosulphate of iron.
Professor Tyndall proved diamagnetic polarity by placing two bismuth bars within two vertical coils or spirals of insulated copper wire, through which electric currents were transmitted from a galvanic battery, and caused to act upon a steel magnet freely suspended without the spirals. Now, when the excited magnetism is merely by induction, the electric current, being momentary, only causes a shock or momentary deviation in the magnet, which returns to its original position when the current ceases. When, on the contrary, the magnetism is permanent, the suspended magnet does not return to its original position when the current ceases. In Professor Tyndall’s experiment the deviation was permanent, and it was equally so when a bismuth bar was freely suspended and the cores within the spirals were steel magnets. Had the effect been from currents induced in the mass of the bar of bismuth, division of the bar would have stopped them, but the result was the same with powdered bismuth as with the solid mass. Moreover, since the strength of induced currents depends upon the conducting power of the substance, and as the conducting power of copper is forty times as great as that of bismuth, had the polarity been induced and not real, the effect ought to have been forty times greater when copper instead of bismuth cores were put in the spirals, whereas it was scarcely sensible. Besides these proofs, Dr. Tyndall made experiments with eleven different diamagnetic substances, of which water was one, with similar results. He then determined the polarity of twelve paramagnetic bodies by the same method, whence it appeared that the same action which produced a north pole in the paramagnetic bodies produced a south pole in those that were diamagnetic, and vice versâ, whence he concludes that diamagnetic polarity is one of the most firmly established truths of science. It follows from this that, when a man is standing, his head is a north pole and his feet a south, and the top of an iron railing on which he may be leaning is a south pole and the lower end a north. Diamagnetic bodies thus possess a polarity, the same in kind but opposite in direction to that possessed by paramagnetic ones.[[18]] They are both dual powers, and the two diamagnetic forces like the two paramagnetic being coexistent, simultaneous, and mutually dependent, there can be no doubt that the diamagnetic forces also are represented, or rather consist of curved and closed lines of force passing through the interior of the substance. Dr. Tyndall has proved that the attraction of iron, and the repulsion of bismuth, are as the square of the electro-magnetic current producing them, and that diamagnetic substances are capable of induction.
The molecular structure of substances freely suspended between the poles of a magnet has a decided effect upon the position they assume.
It has already been mentioned that the optic axis is a symmetrical line in a doubly refracting crystal in which there is no double refraction, and that in some crystals there are two such symmetrical lines. Now, Professor Plücker of Bonn discovered, when such crystals are submitted to powerful magnetic influence, that the single optic axis in the one, and the resultant or mean line between the double optic axes in the other, set diametrically or at right angles to the line of magnetic force; and so powerful did the Professor find the action of magnetism on crystalline form, that the mineral cyanite, when suspended, arranges itself so definitely with regard to terrestrial magnetism, that it might be used as a compass needle.
Dr. Faraday afterwards observed that amorphous substances, cut in the form of a sphere, have no tendency to set or be attracted or repelled in one direction in preference to any other; but if the sphere be formed of a crystallized substance, it is a general fact that, whether it be paramagnetic or diamagnetic, it is more powerfully attracted or repelled in one direction than in any other—a property named by Dr. Faraday magnecrystallic action. For example, a sphere of calcareous spar, which is a diamagnetic crystal, is most strongly repelled in the direction of its principal optic axis, and least strongly in the direction of its least axis. In a sphere of carbonate of iron, which has exactly the same crystalline form and is highly paramagnetic, the line which in carbonate of lime sets equatorially, in this case sets axially, and more strongly in that direction than in any other. The law according to which the attraction of the carbonate of iron increases from the least to its greatest or principal optic axis, is precisely the same as that according to which the repulsion of the calcareous spar increases from the least to the principal optic axis. These relations are not altered by the immersion of the spheres in liquids of either magnetism. Dr. Faraday observed that a line at right angles to the planes of principal cleavage in crystals takes the axial position, and on that account he called it the magnecrystallic axis. Its position was proved by MM. Tyndall and Knoblauch to depend upon the general fact, that the mass is most strongly repelled in the direction of the planes of principal cleavage, and that the elective position of crystals depends more upon the direction of these planes with respect to the electric force, than upon the optic axis. The planes of principal cleavage set themselves equatorially in diamagnetic, and axially in paramagnetic substances: it was thence inferred that the phenomena offered by crystals in the magnetic field is a particular case of the general law, that the superior action of magnets upon matter in a particular direction is due to the particles of the body being closer together in that direction than in any other: in short, the line of maximum density; the force exerted being attractive or repulsive according as the particles are paramagnetic or diamagnetic.