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XXII. Electricity and Galvanisn.

540. If a piece of glass, or sealing-wax, be rubbed on a piece of dry woollen cloth, or silk, and instantly held over any light substances, they will be attracted towards it, raised on an end, and otherwise put in motion.

The power thus excited is called electric: and if the experiment be made in the dark, the glass and the wax will exhibit faint signs of light; which light is called the electric fire or fluid.

541. If the glass be of larger dimensions, and turned rapidly round by a winch and a weel, instead of being rubbed backward and forward with the hand, and be provided with a piece of silk to rub against it during its rotation, streams and large sparks of fluid fire will be elicited, which will fly round the glass, attract light bodies, and produce a pungent sensation.

Obs. 1.-By attracting light bodies, it is meant, that light bodies move towards the electrified body; but it is to be presumed that that which is called attrac tion, is in truth some impulse from without or beyond the light body. The mechanical action of attraction supposed to exist between distant bodies, seems to be an absurdity, and ought to be expunged froin science The term repulsion is even more absurd than attrac tion.

Sir Richard Phillips in his Essays, page 92, proposes to substitute for the active verbs attract and repel, the passive ones accide and discede; and their nouns accision and discession, as implying the mere facts of going together and separating.

542. This glass, its cushion of silk, wheel, &c. are called an electrical machine. The fluid, or power produced by it, is one of the most wonderful in nature.

It is found, that it will pass along some bodies, and not along others; that it may be received and diffused

by sharp points; that a superabundance of it, in onc 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.

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543. The bodies, over which it passes freely, are all animals, mo 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 distances.

All these bodies, therefore, are called non-conductors.

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 tkunder 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 niineral 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 mixture 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.

A GALVANIC TROUGH,

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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 con. veying 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 : fragmenta of diamond, and points of charcoal and plumbago, rapidly disappeared, and seemed to evaporate in

Such are the decomposing powers of electricity, that not even insoluble compounds are capable of resisting their en

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