The man of the globe contains various sources of magnetism [alsoser); and since a magnetic needle freely suspended acquires a determinate position, it follows from this law that the magnetism at the south extremity S is Boreal, that is, of the same name as the terrestrial magnetism which is predominant in the northern hemisphere, being repelled therefrom; and the magnetism at the north extremity N is for a like reason Austral. Tho law of magnetic force at different distances is expressed by the Inverse square of the distance : one mode of verifying this law is by observing the time. of the oscillations of a small fine wire, suspended in a plane perpendicular to the magnetic meridian (in order to neutralise the magnetising Influence of the earth), and subjected to the action of a powerful magnet.
We can, by combining these laws, explain the manner in which soft iron, cobalt, and nickel are attracted or suspended by a magnet. These metals, when chemically pure, contain both the mistral and boreal magnetism In a combined state, in consequence of their want of coercive power. When a piece of soft iron is brought near the pole N, which contains the austral fluid, the austral magnetism of the iron is repelled to the farther extremity, and the boreal attracted to the nearer extremity of the iron relative to the point N : this disposition of tho fluid takes place immediately, and the law of force above announced relative to the distances causes the attraction of the fluid at N, on the boreal fluid of the soft Iron, to exceed Its repulsion on the austral, which is more remote from N : the total effect, in virtue of this excess, is therefore Leceeearily attractive. NVhen the iron however is removed from this influence, Its natural magnetisms again recombine. This will not be the care if, instead of soft Iron, we use hardened iron or steel : the decomposition of the natural magnetism. takes place with greater didiculty, in consequence of the coercive power which protects their actual disposition ; but if we use a powerful magnet at one extremity of a steel needle, or, which Is more effectual, a pair of strong magnets at both extremities, the north pole of one and the south pole of the other being brought in contact with the needle, the decomposition will be partially effected, and will likewise be retained by the seine coercive power which opposed its development ; and agreeably with the magnetic laws of repulsion and attraction, that point of the needle in contact with the south polo will become a north polo of the needle, and the other a south 'polo. This method of producing magnetism is liable to
the objections both of producing feeble magnetism and also producing consecutive points.
The quantities of the austral and boreal magnetie fluids in all mag netic bodies are equal; for when we bestow magnetic qualities on Iron or steel by the influence of loadatones, hammering, sudden cooling in the magnetic meridian, &c., no new magnetism is communicate]; but the natural maguctisms, which previously neutralised each other, aro now decomposed. Again, if a magnetic needle be freely suspended by its centre of gravity, the action of terrestrial magnetism produces no linear motion, but only imposes a direction on the magnetic axis : now all the boreal fluid in the globe attracts all the austral fluid of the needle, and rice rend, while the like fluids in • both repel ; hence a motion of progression would be generated, unless the resultant of the repulsive forces on the needle was exactly equal and of an opposite direction to the resultant of all the attractive forces; and the rotatory motion of the needle shows that the points of application of these forces are different ; but the intensity of terrestrial magnetism may be regarded as uniform throughout the extent of the needle, and its direction parallel. In order therefore that the resultants should be equal and contrary, the sums of the boreal and austral forces of the needle must be equal. In this respect magnetism resembles the natural electricities of all substances.
The development of magnetism in bodies, whether by terrestrial action or the influence of loadstones, is analogous to the decomposition of the natural electricities in aisystem of conducting bodies separated by non-conductors under the influence of an external body and their own natural action : hence when magnetism is communicated by a loadatone, oven when in contact, the latter loses none of its own mag netism, as it acts solely by influence; whereas in conducting electrised bodies, contact will communicate electricity : the coercive force of magnets therefore extends even to their surfaces. In fact the reaction of the substance magnetised by influence tends to a further decompo sition of the fluids of the magnetising body, and this gives it greater energy, unless when it is magnetised to saturation, that is, when the internal magnetic forces are equal to the coercive power ; for then any further development of the fluids would be only temporary, and a reunion would take place immediately.