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 Dr. Faraday for the wonderful discovery of the heating power of the magnetic fluid: there is no indication of heat either All kinds of from the animal or thermo-electricities. 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 The needle has been deflected magnetic influence alone.

by all in the same manner, and magnets have been made by all according to the same laws. Ordinary electricity was long supposed incapable of deflecting the needle; M. Colladon and Dr. Faraday however have proved that, in this respect also, ordinary electricity agrees with voltaic, but that time must be allowed 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,— Dr. Faraday accomplished its decomposition, and Dr. Ritchie its composition, by means of magnetic action; and M. Botto of Turin has shown the chemical effects of the thermo-electricity in the decomposition of water, and some other substances. 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 common to the ordinary voltaic, and magnetic fluids; and Professor Linari, of Sienna, has very

lately obtained both the direct and induced sparks from the torpedo, proving that in this respect animal electricity does not differ from the others. Indeed, the conclusion drawn by Dr. 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 matter in all directions; their velocity is prodigious, and their intensity varies inversely as the squares of the distances. 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. 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 vibration of the compass, show an occult connection between all these agents, which probably will one day be revealed. In the mean time it opens a noble field of experimental research to philosophers of the present, perhaps of future ages.

ETHEREAL

SECTION XXXV.

MEDIUM. COMETS. DO NOT DISTURB THE SOLAR SYSTEM. THEIR ORBITS AND DISTURBANCES. PERIODS OF THREE KNOWN. ACCELERATION IN THE MEAN MOTIONS OF ENCKE'S AND BIELA'S COMETS. THE SHOCK OF A COMET. VELOCITY AND PHYSICAL CONSTITUTION. SHINE BY BORROWED LIGHT. ESTIMATION OF THEIR NUMBER.

The

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 allpervading 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. 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 vapours 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 vapours 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. The light of the comet of the year 1811, which was so brilliant, did not impart any heat even when condensed on the bulb

of a thermometer of a structure so delicate that it would have made the hundredth part of a degree evident. 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 vapour, is so rare, and their transit 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 favourable circumstances, a comet cannot remain longer than two hours and a half at a less distance from the earth than 10,500 leagues. 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 50th 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 motion of these moons. M. Dusejour has computed that a comet, equal in mass to the earth, passing at the distance of 12,150 leagues from our planet, would increase the length of the year