by sharp points; that a superabundance of it, in one place, acts as a repellant in the parts immediately adjoining; and that it has a constant and violent tendency to restore its own equilibrium in all bodies. THE ELECTRICAL MACHINE. 543. The bodies, over which it passes freely, are all animals, most animal and vegetable substances, water, &c.; all which are called conductors of electricity. But it will not pass over glass, sulphur, charcoal, silk, baked woods, or dry woollen substances; nor through air, except by force, in sparks, to short dis tances. All these bodies, therefore, are called non-conduc tors. 544. The power of exciting it, receiving it on points, and confining it to bodies, over which it freely passes, by placing these on bodies, over which it will not pass, gives rise to all the phenomena of practical electricity. Hence a metallic conductor, provided with brasspoints, and elevated on glass-legs, is placed opposite the revolving glass-cylinder, to receive by its points the electric power, which is condensed on the cylinder, but unable to escape on account of its being surrounded only by air, and supported by glass-legs, both which are non-conductors. 545. If the hand, or a metellic knob, be held within three or four inches from this metallic or main conductor, a large spark will escape, which, in the dark, will be forked, and of the colour of lightning. There will also be a snapping noise; which, increased by large quantities, would be likely to produce the noise of thunder. In fact, lightning and thunder are effects of electricity in the clouds. A flash of lightning is simply a stream of the electric power passing from the clouds to the earth; from the earth to the clouds; or from one cloud to another cloud; and thunder is the report, and the echoes of the report, between the clouds and the earth. 546. But the most wonderful effect of the electric fluid, is its power of suddenly contracting the muscles of animals when it violently passes through them from one place to another, to restore its equilibrium. It will not pass through glass; if, therefore, a plate of glass, in the form of a jar, or otherwise, be coated on both sides, with either gold, silver, or tin-foil, and one side be brought into contact with the main conductor, the other side will instantly part with its electricity, and the plate of glass be said to be charged. 547. If one hand be put to the under or outer side of the said charged plate, and the other hand be brought into contact with the other or charged side, the equilibrium of the two sides will be restored through the body; and a violent contraction, or blow of the muscles will be felt, producing a shock peculiar to this operation. The severity of the shock, is proportioned to the size of the plate or jar. When many jars are joined together, and charged in this way, they are called a battery; and some batteries have been made so powerful, as to kill an ox, melt gold, and produce all the surprising phenomena of real lightning. 548. Philosophers amused themselves, for a cen tury, with experiments on the electrical apparatus ; but a new mode of exciting this power, was discovered by Galvani; and the experiments made in his way, are called Galvanism. It is found, that there are two classes of conductors: -perfect, as the metals; and imperfect, as water and the mineral acids; and if these are laid alternately, two perfect and one imperfect, or two imperfect and one perfect, the two ends or sides, will constantly produce an electric shock. Instead, therefore, of the glass-cylinder, conductor, coated jar, &c., used in electrical experiments, the Galvanic pile, or trough, is now preferred. Obs.--The common exhibition of electrical effects, is in attractions and repulsions, in which masses of matter are concerned, but there are other effects, in which, the changes that take place, operate, in a manner, in small spaces of time imperceptibly; and in which, the effects are produced upon the chemical arrangements of bodies. If a piece of zinc and a piece of copper be brought in contact with each other, they will form a weak electrical combination; of which the zinc will be positive, the copper, negative: this may be learnt by the use of a delicate condensing electrometer; or by pouring zinc filings through holes, in a plate of copper, upon a common electrometer; but the power of the combination may be most distinctly exhibited, in the experiments, called Galvanic experiments: by connecting the two metals, which must be in contact with each other, with a nerve and muscle in the limb of an animal recently deprived of life,-a frog, for instance; at the moment the contact is completed, or the circuit made, one metal touching the muscle, the other the nerve, violent contractions of the limb will be occasioned. 549. The Galvanic apparatus consists of a narrow trough of earthenware, with grooves at certain distances, into which are slid alternately, plates of zinc and copper; and between each division, is poured a mix, ture of acid and water. If one hand be put to the plate of copper at the end of the trough, and another be put to the plate of zinc at the other end, a smart shock will be felt, and will be continued for a great length of time. Obs, 1.-The above represents an earthenware trough with its plates of separation, consisting, alternately, of copper and zinc. A wire is fixed at each end, for the purpose of conveying the stroke to the flat plate, or for any other desired purpose. : 2. The most powerful combination that exists, in which number of alternations is combined with extent of surface, is that constructed by the subscriptions of a few zealous cultivators and patrons of science, in the laboratory of the Royal Institution. It consists of two hundred instruments, connected together in regular order, each composed of ten double plates, arranged in cells of procelain, and containing in each plate thirty-two square inches: so that the whole number of double plates is 2,000 and the whole surface 128,000 square inches. This battery, when the cells were filled with 60 parts of water mixed with one part of nitric acid, and one part of sulphuric acid, afforded a series of brilliant and impressive effects. When pieces of charcoal, about an inch long and one sixth of an inch in diameter, were brought near each other (within the thirtieth or fortieth part of an inch,) a bright spark was produced, and more than half the volume of the charcoal became ignited to whiteness; and, by withdrawing the points from each other, a constant discharge took place through the heated air, in a space equal at least to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle. When any substance was introduced into this arch, it instantly became ignited; platina melted as readily in it as wax in the flame of a common candle: quartz, the sapphire, magnesia, lime, all entered into fusion: fragments of diamond, and points of charcoal and plumbago, rapidly disappeared, and seemed to evaporate in it. Such are the decomposing powers of electricity, that not even insoluble compounds are capable of resisting their en ergy for even glass, sulphate of barytes, fluor spar, &c. when moistened and placed in contact with electrified surfaces from the Galvanic apparatus, are slowly acted upon, and the alkaline, earthy or acid matter, carried to the poles in the common order. Not even the most solid aggregates, nor the firmest compounds, are capable of resisting this mode of attack; its operation is slow, but the results are certain: and sooner or later, by means of it, bodies are resolved into simpler forms of matter. DAVY. 550. It is ascertained, that during these shocks, an oxidation of the metallic plates takes place; and, after their surfaces become tarnished, the shock diminishes; but, on being wiped, its force is renewed. A combination of these troughs forms a galvanic battery, the force of which has produced the most surprising effects and, by disturbing the close affinity of the constituent parts of many bodies, has led to the analysis of substances, hitherto deemed simple and elementary. Obs. Some fishes, as the torpedo, the gymnotus electricus, and the silarus electricus, when touched, communicate shocks to the human body like those of electricity but as there is no circuit for the fluid, in these cases, no adequate solution has yet been found of this strange phenomenon. : 551. Since the identity of lightning and electric matter has been ascertained, philosophers have been led to seek the explication of aerial meteors in principles of electricity and there is no doubt, that the greater part of them, and especially the aurora borealis, are electrical, or gaseous phenomena. It has been observed, that the aurora borealis produces a very sensible fluctuation in the magnetic needle; and that the flashes have been attended with various rumbling and hissing sounds. Now black, and deep, the night begins to fall, Who then, bewilder'd, wanders through the dark, Perhaps, impatient as he stumbles on, Struck from the root of slimy rushes, blue, |