Magnetism, from its effects on the human body, can be scarcely an object of our attention; yet, as folly and fraud have brought it forward in a conspicuous view, it will be necessary to ascertain its real nature, and the advantages which may have certainly been derived from it in medicine. Add to this, that quackery is too fascinating to the human mind to be long without an object, and the exploded artifice of to day may, at a future lime, revive in a new form - alter et idem.

Magnetism is strictly the power by which an iron ore attracts or repels a piece of iron, according to the point presented, or attracts only a rude mass. The iron ore, or magnet, can communicate this property to a piece of soft malleable iron, so as to make it much more powerful than any natural magnet. Iron, also, which has long stood in one position, acquires at either end its power of attraction or repulsion. This property is confined to iron, though cobalt and nickel are suspected of having a small degree of magnetism; and to posses the power of attraction, iron must be in a soft, malleable state. When oxided in a slight degree, the magnetic power is weakened; when hardened, or in the state of steel, it receives this power in a small proportion. It is equally necessary that its structure (may we be permitted to call it organization?) should be entire; for a magnetic wire, twisted round a stick, does not lose its virtue, while it has not been so much bent as to destroy its elasticity; but, when it can no longer restore its former shape, the magnetism is lost. A smart blow will sometimes destroy, or in turn, give this power.

Two important errors on this point must be corrected: the one, already noticed, that the magnetic needle, freely suspended, does not lie in the direction of north and south, but a little on the east or west, ac-cording to its"variation." This fact is repeated to add, that a needle only becomes spontaneously magnetic by lying in the magnetic meridian. Another error is, that the magnetic influence resides in the earth. In fact, it seems to pass over its surface; for it is much less obvious in caverns than on the earth. The opinion of its cause being one great magnet at the centre of the earth is, of course, without foundation.

It has been usual to suppose the attraction and repulsion of magnetic bodies to be owing to two different antagonizing fluids. This opinion, supported by the authority of AEpinus, Coulomb, and Hauy, should not be rashly rejected. It is, however, seemingly borrowed from the two electricities; and, as we have found that the electrical phenomena with which we are in this work engaged, might be explained on the supposition of a single fluid, so we think the phenomena of magnetism equally compatible with one fluid. Some analogy has been observed between magnetism and electricity; but, if there is any resemblance, magnets are like the electrics per se. Instead of iron being peculiarly attractive of the magnetic fluid, it appears to be the only body which resists it. From this resistance the phenomena apparently arise. Electrics per se equally resist the electrical fluid; but these, if powdered, are changed into conductors. Powdered magnets are still magnetic.

Magnetism differs from electricity in being influenced by very different laws. Magnetism attracts large bodies, electricity small ones; magnetic attraction is constant; electrical variable: the former limited to about two feet, the powers of the latter are unlimited. The magnetic power is also permanent for ages, if not destroyed by an opposite current of a similar nature, as laying two magnets together, with the north poles contiguous, and is not, or very slightly, affected by moisture, water, and oils, nor at all influenced by an electrical atmosphere. A magnet in action may be electrified without disturbing that action, which is also equally active in a vacuum. Heat also diminishes the magnetic power, and entirely destroys it when the iron becomes red; but it is again recovered on cooling.

These observations are sufficient to show that, if magnetism has no power of its own, little medical effect is to be expected from any fancied analogy to elecricity; and, indeed, magnetism has no analogy to any part of our system, except the small quantity of iron in the blood, which is too much diffused to be influenced by it. In fact, magnetism has no effects but in the promises of the artful, and the delusions of the credulous.

Not many years have elapsed since what is called animal magnetism was supposed to cure every disease, and to free the mind from the trammels of the body, the load of earth which confines its active excursions, enabling it to pervade, at will, through distant regions, unlimited by time or space. This imposition has had a variety of professors in different countries; and, at one time, seems to have fascinated minds even of a superior order. It affected chiefly the imagination; and the delusion was, in general, confined to the female world, and the weaker classes of mankind. An hysteric paroxysm was produced, and the wanderings of a disturbed imagination were received as the dictates of inspiration. In these wanderings, medical questions were proposed and answered; but all the answers, like those of the ancient oracles, were vague and indecisive. The gesticulations of the professors were directed to particular parts, and supposed to remove the complaints of those organs. While the fancy was inflamed, the effects were thought supernatural. When that cooled, the power lost its influence. The professors have published their secret, whicli is a strange mixture of absurdity and fanaticism. They are to powerfully excite the attention, to will an end, with views strictly benevolent, moral, and religious. ' They were not conscious of any means, and this all-powerful influence was to be excited by the volition of the weakest, meanest, sometimes the most infamous, of mankind. The bubble is now burst, and the experience of this age will, for a time, prevent its revival.