serves that these experimens are only cases of the dischage 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 al

lowed for its action. It deflected the needle, whether 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 thermo-electricity 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 chem

ical 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 development 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 development of electricity by heat, the influence of heat on magnetic bodies, and that of light on the vibrations of the compass, show an occult connection 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.

SECTION XXXVI.

In considering the constitution of the earth and the fluids which surround it, various subjects have presented themselves to our notice, of which some, for aught we know, are confined to the planet we inhabit; some are

common to it and to the other bodies of our system; but an all-pervading ether probably fills the whole visible creation, and conveys, in the form of light, tremors which may have been excited in the deepest recesses of the universe thousands of years before we were called into being. The existence of such a medium, though at first hypothetical, is nearly proved by the undulatory theory of light, and rendered all but certain, within a few years, by the motion of comets, and by its action upon the vapors of which they are chiefly composed. It has often been imagined that, in addition to the effects of heat and electricity, the tails of comets have infused new substances into our atmosphere. Possibly the earth may attract some of that nebulous matter, since the vapors raised by the sun's heat, when the comets are in perihelio, and which form their tails, are scattered through space in their passage to their aphelion; but it has hitherto produced no effect, nor have the seasons ever been influenced by these bodies. In all probability, the tails of comets may have passed over the earth without its inhabitants being conscious of their presence.

The passage of comets has never sensibly disturbed the stability of the solar system; their nucleus, being in general only a mass of vapors, is so rare, and their transits so rapid, that the time has not been long enough to admit of a sufficient accumulation of impetus to produce a perceptible action. Indeed, M. Dusejour has proved that, under the most favorable circumstances, a comet cannot remain longer than two hours and a half at a less distance than 10300 leagues from the earth. The comet of 1770 passed within about six times the distance of the moon from the earth, without even affecting our tides; and as the moon

has no sensible influence on the equilibrium of the atmosphere, a comet must have still less. According to La Place, the action of the earth on the comet of 1770 augmented the period of its revolution by more than two days; and if comets had any perceptible disturbing energy, the reaction of the comet ought to have increased the length of our year. Had the mass of that comet been equal to the mass of the earth, its disturbing action would have increased the length of the sideral year by 2h 53m; but as Delambre's computations from the Greenwich observations of the sun, show that the length of the year has not been increased by the fraction of a second, its mass could not have been equal to the part of that of the earth. This accounts for the same comet having twice swept through the system of Jupiter's satellites without deranging the motions of these moons. Dusejour has computed that a comet, equal in mass to the earth, passing at the distance of 12150 leagues from our planet, would increase the length of the year to 367d 16h 5m, and the obliquity of the ecliptic as much as 2o. So the principal action of comets would be to alter the calendar, even if they were dense enough to affect the earth.

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Comets traverse all parts of the heavens; their paths have every possible inclination to the plane of the ecliptic, and, unlike the planets, the motion of more than half of those that have appeared have been retrograde. They are only visible when near their perihelia; then their velocity is such, that its square is twice as great as that of a body moving in a circle at the same distance, they consequently remain a very short time within the planetary orbits; and all the conic sections of the same focal distance sensibly coincide, through a small arc on each side

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