200. Mr. Singer in the following experiment has availed himself of the discovery of Cavallo, who observed that some mineral colors are affected by passing over them the electrical discharge. Color, says he, both sides of a card with vermilion, and place it upon the table of Henley's discharger; one of the wires should be beneath the card, and the other in contact with its upper surface; the distance of the points of the wires being one inch. If a charge be now sent through the wires, the fluid will pass from the positive wire across the surface of the card to the part over the negative wire, and will there perforate the card in its passage to that wire. The course of the fluid is indicated by a black line on the card, reaching from the point of the positive wire to the perforation; and by a diffused black mark on the opposite side of the card around it, and next the negative wire. These effects are pretty constant, the black line always appearing on the side of the card which is in contact with the positive wire, and the perforation being near the negative wire.

201. But the most satisfactory exhibition of the course of the fluid from the positive to the negative conductor is afforded in the next experiment, which was the contrivance of Mr. Singer, and which he himself considered as removing all difficulties on the subject. It has, observes he, been long known that a light floatwheel, made by inserting several vanes of card in the periphery of a cork that is made to turn freely on a pin or centre, will be put in motion by presenting it to an electrified point; and the motion of the wheel being always from the point, whether that point was positive or negative, has been occasionally urged as an argument for a double current of the fluid; although it is evident, from what has been already stated, that a point either positive cr negative must produce a current, by the recession of the air opposed to it when similarly electrified, by its contact, which is fully adequate to the production of these effects. Conjecturing that the currents of electrified air would not take place in this manner if the points were opposed to each other, the following arrangement was made.

202. A light float-wheel of the above description, being mounted so as to turn freely between two upright wires, was placed on an insulated stem, and introduced between the pointed wires of Henley's discharger, which were placed accurately opposite to each other, and at the distance of rather better than an inch from the upper vanes of their respective sides. On connecting one of the wires with the positive conductor of the machine, and the other with the negative conductor, and turning the machine, the wheel will move in a direction from the positive to the negative wire. On reversing the connexions, so that the wire which was negative may become positive, and that which was positive be made negative the motion of the wheel will be reversed; for it will still move from the positive to the negative wire, thus proving that the electricity moves in that direction. A representation of this wheel, with the method of using it, is given in fig. 3, where A is the floatwheel placed on the universal discharger B, the

wires of which, C, C, are directed against the floats of the wheel.

EXPANSIVE POWER OF THE FLUID. 203. The influence of the electric matter in expanding bodies through which it is sent, may be clearly illustrated by these experiments. Place a small card, or the cover of a book, against the outer coating of a charged jar, exposing about a square foot of coated surface; put one end of the discharging rod against the card, and bring the other to the knob of the jar; the charge will pass through the card and perforate it, producing a small bur on the side next the discharging rod, and a larger one on the side which was in contact with the coating of the jar. In the same manner, by using the more powerful charge of a battery, a perforation may be made through a quire of paper or a thin unbound book; and, if either of these be freely suspended between the balls of the universal discharger, no motion of the paper will be produced, but the charge will pass through it without in the least disturbing it, for the same reason that a musket ball will pass through a door without causing it to turn on its hinges, although, under the same circumstances, a very slight force would be sufficient to move it.

204. The effects produced on the card in the preceding experiment, as well as some others that have resulted when the experiment has been varied, have led some electricians to suppose that they might be viewed as indications of the course of the electric matter, and as affording no obscure proof of the existence of two fluids; but Mr. Singer has, in our opinion, satisfactorily shown that they are produced solely by an expansion of the paper.

205. The following illustration of this influence is attributed to Mr. Lane, the inventor of the discharging electrometer. Roll up a piece of soft pipe-clay, in the form of a small cylinder, and insert in it two wires, so that their ends, without the clay, may be about one-fifth of an inch from one another. If a charge be sent through this clay, by connecting one of the wires with the outside of a jar, and the other with the inside, it will be inflated by the shock passing between the two wires, and, after the explosion, will appear swelled in the middle. If the charge sent through it is too strong, and the clay not very moist, it will be broken by the explosion, and the fragments scattered in every direction. To make this experiment with a little variation, take a piece of the tube of a tobacco-pipe, about one inch long, and fill its bore with moist clay; then insert in it two wires, as in the above rolled clay, and send a charge through it. This tube will be burst by the force of the explosion, and its fragments will be scattered about to a great distance. If, instead of clay, the above-mentioned tube of the tobacco-pipe, or a glass tube, which will answer as well, be filled with any other substance, either electric or non-electric, inferior to metal, on making the discharge it will be broken to pieces with nearly the same force.

206. The expansion of fluids by electricity is truly remarkable, and often productive of some singular results. When the charge is strong, no

glass vessel can resist its force. Beccaria placed a drop of water between two wires in the centre of 3. solid glass ball of two inches in diameter; on passing a shock through the water the ball was dispersed with great violence. Mr. Morgan succeeded, by the same means, in breaking green glass bottles, filled with water, when the distance between the wires that conveyed the spark and the sides of the glass was upwards of two inches.

207. A single spark may be made to perforate a strong glass tube by the following simple process: fill a small phial with olive oil, and insert into it a pointed wire bent at right angles, so that by sliding through a cork fixed in the neck of the phial, the point of the wire may rest against any part of the inside beneath the oil: attach the phial by its wire to the conductor of the machine, and bring the knuckle or a brass ball near the outside of the phial, opposite to the point of the wire within it; a spark will pass from the point to the knuckle, and make a small hole in the glass. By varying the situation of the point, many such perforations may be made in the glass.

208. Of this very singular experiment Mr. Singer offers the following rational account. The point, he says, serves as an internal coating to a very small portion of the glass; and the charge being prevented from extending, by the surrounding oil, the whole power of the machine is concentrated in that point, and consequently soon overcomes its resistance. Similar effects will always result when a large quantity of electricity is suddenly transferred to a comparatively limited surface.

209. Make a small mortar of ivory, with a cavity of half an inch diameter and an inch deep; insert two wires through the sides of the mortar, so that their points within the cavity may be separated by an interval of of an inch; fit a cork cap so as to close the aperture as accurately as possible without friction: when a strong charge is passed through the wires, the air within the mortar will be suddenly expanded, and the cork projected to a distance with some violence.

210. Satisfactory and striking as the preceding experiments are, they appear inferior to the following in making experiments on the effects of explosions of electricity sent through metallic bodies, Dr. Priestley found that a chain which he had used as the medium of conveying the charge was shorter after being used than it was before. This led him to try the effect of a strong charge on a definite length of chain: the charge was sent from sixty-four square feet of coated surface, through twenty-eight inches of chain, which, on being measured again, was found to be contracted a quarter of an inch in its whole length. 211. This experiment was repeated by Mr. Nairne with a piece of hard-drawn iron wire ten inches in length, and of an inch in diameter. Through this wire he discharged twentysix feet of coated surface nine times; after the sixth and the ninth time the wire was measured, and it was found to be shortened of an inch after each discharge. By farther discharges sent through it, the wire was shortened 1th inch.

That this contraction arose from expansion appeared from the wire having increased in thickness whilst it was diminished in length.

SOLID BODIES RUPTURED BY ELECTRICITY. 212. The simplest form, perhaps, in which this can be done, is to place on the table of Henley's discharger a piece of dry writing-paper, and pass over it, by means of the pointed wires, a powerful charge from a large jar; if the wires be placed at about two inches asunder, and so as to touch the ends of the paper, it will be torn in pieces. If, instead of the paper, a number of wafers be placed in the circuit, they will be curiously dispersed, and many of them broken to pieces.

213. Make two small holes in the opposite ends of a piece of oak, of about half an inch long and a quarter of an inch thick; introduce two wires into the holes, so that their extremities within the wood may be rather less than a quarter of an inch distant; on passing a strong charge from one wire to another, the wood will be split. Several other substances that are imperfect conductors, such as loaf-sugar, marble, &c., undergo similar injuries, when introduced into an electrical circuit; and even the hardest bodies, and the most perfect non-conductors, such as glass, are perforated or broken by a strong charge.

214. Lay a piece of plate-glass of about an inch square, and half an inch thick, on the table of the universal discharger, or screw it very tightly within the press of the same; set the prints of the sliding wires opposite to each other, and against the under surface of the glass, so that the charge may pass along that surface. If now a strong charge be passed in this way, the glass will be broken into small fragments, and some of it reduced to an impalpable powder.

215. The following experiment affords a beautiful, though rather an expensive, illustration of the same principle. Charge a very large jar, and connect its external coating with that of one ten or twelve times smaller; form a communication between their internal coatings by the discharging rod, and the small jar will be broken in pieces, by the strength of the quantity of electricity thus suddenly transferred to it.

INFLAMMABLE SUBSTANCES KINDLED BY
ELECTRICITY.

216. Almost all inflammable substances may be kindled by means of electricity, but those generally selected for the purpose of experiment are resin, gunpowder, rectified spirit of wine, ether, and hydrogen gas.

217. The common method of kindling resin by the electric spark is to pulverise it, and dust the powder on some dry cotton wool. Thus, if a small quantity of flax, or of cotton wool, be loosely tied on one of the knobs of the discharging rod, and a little finely-powdered resin dusted on it, and a jar be discharged by bringing the end of the rod thus prepared in contact with the knob of the jar, the charge will pass through the flax, or wool, and in so doing will melt and ignite the resin, and set the whole on fire. A very neat contrivance for giving this experiment

a better effect is represented by fig. 4. A is a mahogany board, six inches in length, three in breadth, and half an inch in thickness. B is a glass pillar fixed in the middle of A, and supporting a piece of wood C, which is three inches long, an inch and a half broad, and about three quarters of an inch thick. In each end of this piece there is a small screw-ring: the extremities of these screws just touch a wire proceeding to each of them from two small brass knobs inserted at the ends of a shallow groove on the upper side of C; these rings serve to hook the chains on when the instrument is used. On the back part of C stands the perpendicular piece D, in the top of which is fixed the brass pin E. To use this for the purpose of igniting resin, it is only necessary to dust the cotton with powdered resin, and hang it on the pin E, letting the lower part of it reach down to the piece C. Connect one end of C with the inside of a jar and the other end with the outside; discharge the jar, and the fluid in passing between the two small brass knobs will inflame the resin and kindle the cotton.

218. But the inflammation of resin is rendered still more striking by the following experiment: let a flat porcelain dish be filled with water, and on the surface of the water strew a quantity of finely powdered resin; place two wires at the opposite sides of the dish, having their ends near the surface of the water, and at about the distance of four inches from each other. Pass the charge of a large jar through this circuit, and the resin which forms a part of it will be beautifully inflamed.

219. The firing of gunpowder by the electrical explosion requires considerably more care and address than the firing of resin. It may, however, be effected by the following method: take either a large goose-quill or a small cartridge of paper, and fill it with gunpowder ground very fine; into this insert two wires, one at each extremity, so that their ends within the quill or cartridge may be about one-fifth of an inch from each other: this done, send the charge of a phial through the wires; and the spark between their extremities, that are within the cartridge, or quill, will inflame the gunpowder. If the gunpowder be mixed with steel-filings, it will take fire more readily, and with a very small charge.

220. Rectified spirit of wine, or ether, may be thus inflamed by a single spark from the conductor of the machine when in action: hang to the prime conductor a short rod having a small knob at its end; then pour some spirit of wine, a little warmed, into a metallic spoon; hold the spoon by the handle, and place it so that the small knob on the rod may be about one inch above the surface of the spirit. In this situation, if a spark be taken from the knob, it will set the spirit on fire. This experiment may be varied and rendered very agreeable to a company of spectators. A person standing upon the insulating stool, and communicating with the prime conductor, may hold the spoon with the spirit in his hand, and another person standing upon the floor may set the spirit on fire by bringing his finger within a small distance of it. Instead of

his finger, he may fire the spirit with a piece of ice, when the experiment will seem much more surprising. If the spoon is held by the person standing upon the floor, and the insulated person brings some conducting substance over the surface of the spirit, the experiment succeeds equally well. This experiment is sometimes rendered still more striking in the following manner near the prime conductor of the machine, place on the table three wine glasses; connect the first glass with the conductor by a brass chain, which will reach to the bottom of it; and with it let the second and third be connected by a piece of fine brass wire, bent in the form of the letter A. Fill the first and second glasses with water, and into the third pour a little ether; turn the machine, and with a wire, having a small ball affixed to it, draw a spark from the ether, and it will be immediately inflamed. In this experiment the electric fluid had to pass through two distinct portions of water before it could come to the ether.

221. Hydrogen gas is generally inflamed by the electric spark in the following manner: take a brass cannon, such as is represented in fig. 5, and charge it with hydrogen gas by holding the mouth of it for about half a minute over a stone or glass bottle in which the gas is generated, and then fix the cork tightly into the mouth of the cannon. The person who is to discharge it must now stand on the insulating stool, and, holding in his hand a chain attached to the prime conductor, must, with a wire and ball held in the other hand, communicate a spark to the knob of the cannon: this spark will pass into the interior of the cannon through the glass tube B, and the gas will instantly explode with a loud report, and the cork will be driven out to a considerable distance. This experiment may be rendered more effectual by mixing about onethird of oxygen with the hydrogen gas; or, if oxygen cannot be conveniently procured at the time, the operator may easily contrive to have some atmospheric air in the cannon, which will render the hydrogen gas more explosive.

222. The very great ease with which inflammable air is kindled by even a small spark of electricity seems to have suggested to M. Volta the idea of what he calls his inflammable airlamp, an instrument which appears to have laid the foundation for the patent apparatus for obtaining instantaneous light. This curious instrument is represented in fig. 6, where A is a glass globe for containing hydrogen gas; B a glass reservoir for holding water; and D a stop-cock, by which a communication is formed between the water and the gas. The water passes into the globe through the pipe gg, which is fixed into the top of the reservoir A; at s there is a cock to cut off or open a communication between the air and the jar K. N is a contrivance for holding a wax-taper; L a brass pillar, on the top of which is fixed a ball of the same metal; a is a pillar of glass, with a socket at the top, in which the wire b slides, having a ball screwed on the end of it. F is a cock by which the globe is filled with hydrogen gas, and which afterwards serves to confine it, and what water falls into B and A. To use this instrument,

having filled the globe with gas, and the reservoir A with water, turn the cocks D and s, and the water will fall into the globe, forcing up a quantity of gas, which will rise through the pipe K. If, now, an electric spark be made to pass from the ball m to that marked n, it will set fire to the gas which passes through the pipe K. To extinguish the lamp, first shut the cock s, and then D. The gas is obtained in the usual way, from diluted sulphuric acid and iron filings; and the globe is to be filled in the following manner: Having previously filled it with water, place the foot R in a vessel of water, so that it may be covered, and the bent glass tube, through which the gas is to be introduced, may pass commodiously below the foot. When the gas has displaced nearly all the water, turn the cock F, and the lamp is ready for use.

223. This instrument is sometimes constructed so as to be connected with an electrophorus for the purpose of producing a light at pleasure. In this case the electrophorus is placed in a box beneath the vessel containing the hydrogen gas; from this box a wire passes through a glass tube to the opening of the stop-cock. The cover of the electrophorus must be connected by a silk string with the handle of the stop-cock, so that the same motion that opens the cock, may raise the cover of the electrophorus, and the spark that strikes from it be conveyed by the insulated wire to the stream of gas, which it inflames. This effect will take place every time the stopcock is opened, for the electrophorus will produce sparks for a considerable time, without any fresh excitation; and the quantity of gas consumed at each repetition of the process is very small, so that a light may be obtained above a hundred times before the contents of the reservoir are expended; and it may then be easily replenished.

224. But by far the most remarkable effects of electricity in producing combustion, result from its action on metallic substances. This property of the electric matter seems to have been first observed by the celebrated Dr. Franklin, who made several experiments on the subject. These experiments were repeated and extended by Mr. Kinnersly, and also by Beccaria; and have since then been prosecuted with great accuracy by Mr. Brook, Dr. Van Marum, Mr. Cuthbertson, Mr. Singer, and others. In treating on this part of the subject, we shall, as in the preceding instances, give a course of progressive experiments, which may be made with an appropriate apparatus, from a single jar of a tolerably large size, up to a powerful battery, previously remarking that the whole of the apparatus must be in the very best state in which it can be put, otherwise disappointment, in many cases, and inaccuracy in all, will be the result. 225. The slightest indication of the fusing of metallic substances is observed in the discharge of the Leyden phial: on this occasion, if the discharge be made by means of the common discharging rod, the knob that touches the outside coating will be found to adhere slightly to the jar after the discharge; and this arises from the fusing of a small portion of the tin-foil coating.

226. This may be rendered very obvious by the following experiment :-Charge a large jar, and place a smooth piece of coin between the knob of the discharger and the coating of the jar; when the discharge has been made, the piece of coin will be found slightly adhering to the tin-foil, by its fusion at the point of contact, and will remain so as to require some force to separate them.

227. Dr. Franklin's experiments on the fusing of metals may be thus imitated :—Take three pieces of window-glass, each one inch wide, and three inches long, and place them contiguous to each other, with two narrow strips of goldleaf between them, so that the middle glass may have, on each side of it, a strip of gold, with its ends projecting a little beyond the glass. Let the whole be properly secured within the press of the universal discharger; pass the charge of a large jar through the strips of gold, they will be melted and driven into the glass. The outer pieces of glass are generally broken, but the middle piece is often left entire, and is marked with an indelible metallic stain on each of its surfaces.

228. The colors produced when metals are thus fused, either on glass or paper, are sometimes exceedingly beautiful, and have been in some cases employed in impressing letters, and ornaments of various kinds, on silk and paper. The process observed in such cases, Mr. Singer thus describes in his Elements:- The outline of the required figure is first traced on thick drawing paper, and afterwards cut out in the manner of stencil plates. The drawing paper is then placed on the silk or paper intended to be marked; a leaf of gold is laid upon it, and a card over that; the whole is then placed in a press, or under a weight, and a charge from a battery sent through the gold leaf. The stain is confined, by the interposition of the drawing paper, to the limit of the design; and in this way a profile, a flower, or any other outline figure may be very neatly impressed.'

229. In describing Cuthbertson's electrometer, No. 177, we made some remarks on the fusing of wire, by passing through it the discharge of accumulated electricity; it may be necessary to offer here some general observations on the same subject; and to give some account of the results of the experiments made by those whose names we have already enumerated.

230 For the fusing of wires of different lengths very large batteries were formerly considered as indispensable; but Mr. Singer observes, that, if the wire be sufficiently fine, a single jar, exposing a coated surface of about 190 square inches, will be sufficient to afford an example of the effect. The finest flattened steel wire, sold at the watchmakers' tool shops under the name of pendulum wire, is the best for the purpose; harpsichord wire not being fine enough, excepting where great power is used. Cuthbertson's Balance Electrometer should be invariably used to regulate the charge; the circuit made as short as possible; and the wire to be fused placed in a straight line, and held at the ends between a small forceps made of wire.