most likely follows the ratio of the inverse square of the distance, it must extend indefinitely. It is probable that the moon has become highly magnetic by induction, in consequence of her proximity to the earth, and because her greatest diameter always points towards it. Should the magnetic, like the gravitating force, extend through space, the induction of the sun, moon, and planets must occasion perpetual vibrations in the intensity of terrestrial magnetism, by the continual changes in their relative positions.

In the brief sketch that has been given of the five kinds of electricity, those points of resemblance have been pointed out which are characteristic of one individual power; but as many anomalies have been lately removed, and the identity of the different kinds placed beyond a doubt, by Mr. Faraday, it may be satisfactory to take a summary view of the various coincidences in their modes of action on which their identity has been so ably and completely established by that great electrician.

The points of comparison are attraction and repulsion at sensible distances, discharge from points through air, the heating power, magnetic influence, chemical decomposition, action on the human frame, and lastly the spark.

Attraction and repulsion at sensible distances, which are so eminently characteristic of ordinary electricity, and in a lesser degree, also, of the voltaic and magnetic currents, have not been perceived in either the thermo or animal electricities, not on account of difference of kind, but entirely owing to inferiority in tension; for even the ordinary electricity, when much reduced in quantity and intensity, is incapable of exhibiting these phenomena.

Ordinary electricity is readily discharged from points through air, but Mr. Faraday found that no sensible effect took place from a battery consisting of 140 double plates, either through air or in the exhausted receiver of an air-pump, the tests of the discharge being the electrometer and chemical action, a circumstance entirely owing to the small degree of tension, for an enormous quantity of electricity is required

to make these effects sensible, and for that reason they cannot be expected from the other kinds, which are much inferior in degree. Common electricity passes easily through rarefied and hot air, and also through flame. Mr. Faraday effected chemical decomposition and a deflection of the galvanometer by the transmission of voltaic electricity through heated air, and observes that these experiments are only cases of the discharge which takes place through air between the charcoal terminations of the poles of a powerful battery when they are gradually separated after contact—for the air is then heated; and Sir Humphry Davy mentions that, with the original voltaic apparatus at the Royal Institution, the discharge passed through four inches of air; that, in the exhausted receiver of an air pump, the electricity would strike through nearly half an inch of space, and that the combined effects of rarefaction and heat were such, upon the included air, as to enable it to conduct the electricity through a space of six or seven inches. A Leyden jar may be instantaneously charged with voltaic, and also with magneto-electricity — another proof of their tension. Such effects cannot be obtained from the other kinds, on account of their weakness only.

The heating powers of ordinary and voltaic electricity have long been known, but the world is indebted to Mr. Faraday for the wonderful discovery of the heating power of the magnetic fluid: there is no indication of heat either from the animal or thermo-electricities. All the kinds of electricity have strong magnetic powers, those of the voltaic fluid are highly exalted, and the existence of the magneto and thermo-electricities was discovered by their magnetic influence alone. The needle has been deflected by all in the same manner, and, with the exception of thermo-electricity, magnets have been made by all according to the same laws. Ordinary electricity was long supposed incapable of deflecting the needle, and it required all Mr. Faraday's ingenuity to produce that effect. He has, however, proved that, in this respect, also, ordinary electricity agrees with voltaic, but that time must be allowed for its action. It deflected the needle, whe

ther the current was sent through rarefied air, water, or wire. Numerous chemical decompositions have been effected by ordinary and voltaic electricity, according to the same laws and modes of arrangement. Dr. Davy decomposed water by the electricity of the torpedo, -- Mr. Faraday accomplished its decomposition, and Dr. Ritchie its composition by means of magnetic action; but the chemical effects of the thermoelectricity have not yet been observed. The electric and galvanic shock, the flash in the eyes, and the sensation on the tongue, are well known. All these effects are produced by magneto-electricity, even to a painful degree. The torpedo and gymnotus electricus give severe shocks, and the limbs of a frog have been convulsed by thermo-electricity. The last point of comparison is the spark, which is already mentioned as common to the ordinary, voltaic, and magnetic fluids; and although it has not yet been seen from the thermo and animal electricities, there can be no doubt that it is only on account of their feebleness. Indeed, the conclusion drawn by Mr. Faraday is, that the five kinds of electricity are identical, and that the differences of intensity and quantity are quite sufficient to account for what were supposed to be their distinctive qualities. He has given still greater assurance of their identity by showing that the magnetic force and the chemical action of electricity are in direct proportion to the absolute quantity of the fluid which passes through the galvanometer, whatever its intensity may be.

In light, heat, and electricity, or magnetism, nature has exhibited principles which do not occasion any appreciable change in the weight of bodies, although their presence is manifested by the most remarkable mechanical and chemical action. These agencies are so connected, that there is reason to believe they will ultimately be referred to some one power of a higher order, in conformity with the general economy of the system of the world, where the most varied and complicated effects are produced by a small number of universal laws. These principles penetrate in all directions; their velocity is prodigious, and their intensity varies inversely as the

square of the distance. The developement of electric currents, as well by magnetic as electric induction, the similarity in their mode of action in a great variety of circumstances, but above all, the production of the spark from a magnet, the ignition of metallic wires, and chemical decomposition, show that magnetism can no longer be regarded as a separate, independent principle. That light is visible heat seems highly probable; and although the evolution of light and heat during the passage of the electric fluid may be from the compression of the air, yet the developement of electricity by heat, the influence of heat on magnetic bodies, and that of light on the vibrations of the compass, show an occult connexion between all these agents, which probably will one day be revealed; and in the mean time it opens a noble field of experimental research to philosophers of the present, perhaps of future ages.

EXPLANATION OF TERMS.

Electro-Magnetism The science which determines the reciprocal action of electricity and magnetism.

Electro-Dynamics - The science of the motion and reciprocal motion of electric currents.

Galvanism - Electricity perpetually in motion, and produced by chemical action.

Galvanic Battery- An instrument for producing galvanic electricity, constructed of alternate layers of two metals and a fluid. Helix — A curve like a corkscrew, whose turnings may either be circular or elliptical.

Magnetic Induction - The effect of magnets to excite magnetism in bodies near them.

Poles of a Magnet-Points in a magnet where the intensity of the magnetic force is a maximum; one of these attracts and another repels the same pole of another magnet.

Rotation The motion of a body round an axis.

Tangent-A straight line touching a curve in one point.

[From the Knickerbocker, June 1837.]

ELECTRO-MAGNETISM.

Ir was the illustrious KEPLER, if we recollect rightly, who, half piqued at finding that one of his attempts to explain the motions of some planet had been labour lost, compared Dame Nature to the coquette Galatea, in one of Virgil's Eclogues. The nearer she is approached, the more wayward, capricious, and provoking, are her escapadas :

'Fugit ad salices et se cupit ante videri.'

If this be true of astronomical science, it is a truth a thousand times more frequently enforced upon the votary of that class of physical sciences, of which electricity and galvanism form so prominent and interesting a field, both for study and discovery. The very subject-matter of his researches is more like the idea of a 'spiritual essence' than actual bonâ fidê material entity. It is a kind of invisible tertium quid, which baffles all human tests of materiality. Grant, as some philosophers will have it, that it is merely a property of matter,' and that it has none of the ordinary characteristics of material substances yet the difficulty is but increased. If we find it so impossible to believe that matter could travel through matter, with the mysterious velocity with which electricity is impelled; if we cannot conceive of matter which is so subtile as to elude all our senses to penetrate the most solid substances, and to be known only by its effects when in motion, how can we conceive of a mere 'property of matter' of which matter may be deprived in one part, while it is accumulated in another? Of which, in fact, matter, organic or inorganic, is equally unconscious, until a change in its distribution is effected, that developes its latent energy? To call it a mere property of matter, then, like extension, does not seem either very correct in expression, or philosophical in principle.