in the latter; a portion of this fluid must, therefore, be driven from the side adjacent to the first body, towards the remoter side. The adjacent side will thus be rendered negative; the remoter side, positive. But this will take place to a certain extent only; for there is a limit at which the repulsion of the fluid accumulated at the remote end will just balance the repulsion of the fluid in the electric, added to the attraction of the under-saturated matter, in the near end; and when the limit has been attained, the flow of electric fluid from the near to the remote end of the body will cease, and an equilibrium will be established. Experiment fully confirms this theory, as may be seen by bringing a cylinder of metal of some length, with rounded ends, near an electrified globe of glass, taking care that it be not sufficiently near to receive any quantity of electricity by transferrence. By means of the electrometer of Coulomb, we perceive that the part of the conductor nearest to the electric is negative, and the part most remote is positive; while, about the middle of the cylinder, the body is in a neutral state. The electricity is found to diminish as we proceed from either extremity towards this point of neutrality. These remarkable effects are solely the result of the action of electricity at a distance; for they take place in an equal degree, whatever non-conducting substance may be interposed between the bodies exerting this influence on one another. But in an experiment, where the acting body, instead of being an electric, is a conducting body, the electrical state which the globe induces on the cylinder must react upon its own electricity. The negative electricity, that is, the under-saturated matter at the nearer end of the cylinder, must exert a tendency to induce positive electricity in the globe, and more especially upon the side next the cylinder; that is, it will tend, by its attraction for the fluid, to draw it to that side, and thus render it still more highly positive than it was before. This can only be done at the expense of the other side, from which the fluid must be taken, and which is therefore rendered less charged with fluid, that is, less positive than before. But this new distribution of the electric fluid in the globe, by increasing the positive state of the side next the cylinder, tends to augment its inductive influence on the fluid in the cylinder; that is, to drive an additional quantity of fluid from the negative to the positive end. This must be followed in

turn by a corresponding reaction on the globe, and so on, constituting a series of smaller adjustments, until a perfect equilibrium is established in every part. This reasoning is fully established by experiment. All that is required for its illustration is simply to furnish the metallic globe, insulated and charged with positive electricity, with electroscopes upon its opposite surfaces. No sooner do we bring near to it a conducting body, than the balls of the electroscope, at the side most distant from that body, begin to collapse, while those at the nearer side diverge to a greater degree than before; thus showing the nature of the reflex operation of the induced electricity of the conductor upon the body from which the induction originated. In all the changes thus alluded to, there has been no transfer of electricity from either of the bodies to the other, as is most satisfactorily proved from the circumstance, that the mere removal of the bodies to a distance from one another is sufficient to restore each of them to their original state. The globe remains as perfectly electrified as before; the cylinder returns to its condition of perfect neutrality; and the experiment may be repeated as often as we please, without any variation in the phenomena. This would not be the case, however, however, if the cylinder were divided in the middle, and one or both of the parts were removed separately, while they still remained under the influence of the globe. The return of the electric fluid from the positive to the negative end being thus prevented, each part will retain, after its separation, the electricity which had been induced upon it; the nearer portion will remain negative, the remoter one positive. If the division had been in three parts, the middle part only would have been neutral. It is found by experiment, that the effects of induction on a conductor are augmented by increasing its length; and they become as great as possible, by placing the conductor in communication with the earth, which carries off all the fluid the electrified body is capable of expelling from the nearest end.` A conductor under the influence of induction, between which and the earth a communication has been made, by touching the remote end with a metallic rod held in the hand, possesses but one kind of electricity, namely, the one opposite to that of the electrified body which is acting upon it. The part touched is brought into a state in which it appears to be ncutral, as long as it remains in the vicin

ity of the electrified body; but it really contains less fluid than its natural share; and this will immediately become apparent, if the conductor that has been touched be again insulated, and then removed from the influence of the body producing the induction. This peculiar condition of a body, in which its parts are really undercharged or overcharged with fluid, although, from the action of electric forces derived from bodies in its vicinity, a state of equilibrium is established, and no visible effect results, has been denominated by Biot, disguised electricity. We have hitherto supposed the acting body to be positively electrified; but precisely the same effects would happen with regard to the degree, although opposite as to the species of electricity, if it had been negatively electrified. Our knowledge of the induction of electricity enables us to understand why bodies, between which it takes place, should attract one another. For the action of the adjacent sides, which are brought into opposite electrical states, is greater than the action of those sides which are in the same electrical states, and which are more distant; hence the attractive force always exceeds the repulsive. The most convenient mode of obtaining an accumulation of electricity arising from induction, is by the employment of coated glass, that is, of a plate of glass, on each side of which is pasted a sheet or coating of tin-foil. Care must be taken to leave a sufficient margin of glass uncovered by the metal, for preventing the transfer of electricity from one coating to the other, round the edge of the glass; and all sharp angles, or ragged edges in the coatings, should be avoided, as they have a great tendency to dissipate the charge. The form of coated glass best adapted to experiments is that of a cylindric jar; this is coated, within and without, nearly to the top. The cover consists of baked wood, and is inserted with sealing-wax, to exclude moisture and dust. A metallic rod, rising two or three inches above the jar, and terminated at the top in a brass knob, is made to descend through the cover till it touches the interior coating. The name of the Leyden phial, or jar, is applied to this instrument. It is used in the following manner: the outer coating being made to communicate with the ground, by holding it in the hand, the knob of the jar is presented to the prime conductor when the machine is in motion; a succession of sparks will pass between them, while, at the same time, nearly an equal quantity 39

VOL. IV.

457

of electricity will be passing out from the exterior coating, through the body of the person who holds it, to the ground. The jar, on being removed, is said to be charged; and if a communication is made between the two coatings, by a metallic wire, extending from the external one to the knob, the electric fluid which was accumulated in the positive coating rushes, with a sudden and violent impetus, along the conductor, and passes into the negative coating; thus at once restoring an almost complete equilibrium. This sudden transfer of a large quantity of accumulated electricity is a real explosion; and it gives rise to a vivid flash of light, corresponding in intensity to the magnitude of the charge. The effect of its transmission is much greater than that of the simple charge of the prime conductor of the machine; and it imparts a sensation, when passing through any part of the body, of a peculiar kind, which is called the electric shock. In the construction of the Leyden jar, the thickness of the glass is an important consideration. The thinner the glass, the greater will be the power of taking a charge; but the power of retaining it will be less, on account of the diminished resistance which the glass will oppose to the electricity through it. If the charge be higher than what the jar will bear, the glass will be broken by the violence with which the electric fluid forces a passage through its substance. Another limit to the charge which a jar is capable of retaining, arises from the liability of the electricity to pass from one coating to the other, round the edges of the glass. The deposition of inoisture, also, on the glass, will occasion a spontaneous discharge, since it forms a chain of conducting particles, in the very line which the electricity has a tendency to take. Hence, in order to preserve the uncoated part of the glass in as dry a state as possible, it is usually covered with a layer of sealing-wax, or some other resinous varnish. By uniting together a sufficient number of jars, we are able to accumulate an enormous quantity of electricity: for this purpose, all the interior coatings of the jars must be made to communicate by metallic rods, and a similar union must be established among the exterior coatings. When thus arranged, the whole series may be charged, as if they formed but one jar; and the whole of the accumulated electricity may be transferred from one system of coatings to the other, by a general and simultaneous discharge. Such a combination

of jars is called an electrical battery. For the purpose of making the direct communication between the inner and outer coating of a jar or battery, by which a discharge is effected, an instrument called the discharging rod is employed. It consists of two bent metallic rods, terminated at one end by brass balls, and connected at the other by a joint, which is fixed to the end of a glass handle, and which, acting like a pair of compasses, allows of the balls being separated at different distances. When opened to the proper degree, one of the balls is made to touch the exterior coating, and the other ball is then quickly brought into contact with the knob of the jar, and thus a discharge is effected, while the glass handle secures the person holding it from the effects of the shock. If we wish to send the whole charge of electricity through any particular substance, which may be the subject of experiment, we must so arrange the connecting conductors, as that the substance shall form a necessary part of the circuit of the electricity, as it is termed. With this view, we must place it between two good conductors, one of which is in communication with the outer coating; and the circuit may then be completed by connecting the other conductor with the inner coating, by means of a discharging rod, to one branch of which, if necessary, a flexible chain may be added. VII. In forming arrangements for directing the passage of accumulated electricity, it should be borne in mind, that the electric fluid will, on these occasions, always pass through the best conductors, although they may be more circuitous, in preference to those which are more direct, but have inferior conducting power; and it must also be recollected, that when different paths are open for its transmission along conductors of equal power, the electricity will always take that which is the shortest. Thus, if a person, holding a wire between his hands, discharges a jar by means of it, the whole of the fluid will pass through the wire, without affecting him; but if a piece of dry wood be substituted for the wire, he will feel a shock; for, the wood being a worse conductor than his own body, the charge will pass through the latter, as being the easiest, although the longest circuit. During its transit through the human body, in like manner, the shock is felt only in the parts situated in the direct line of communication; and if the charge be made to pass through a number of persons, who take one another by the hand, and form

part of the circuit between the inner and outer coatings of the jar, each will feel the electric shock in the same manner, and at the same instant; the sensation reaching from hand to hand, directly across the breast. By varying the points of contact, however, the shock may be made to pass in other directions, and may either be confined to a small part of a limb, or be made to traverse the whole length of the body, from head to foot. By accurate experiments it appears, that the force of the electric shock is weakened, i. e. its effects are diminished, by employing a conductor of great length for making the discharge. But it is difficult to assign a limit to the number of persons through whom even a small charge of electricity may be sent, so that all shall experience the shock; or to the distance along which it may be conveyed by good conductors. The abbé Nollet passed an electrical shock through 180 of the French guards, in the presence of the king; and the sensation was felt at the same moment by all the persons composing the circuit. An experiment was made near London, at a time when the ground was remarkably dry, to ascertain if any loss of time accompanied the passage of the fluid, when transmitted through considerable distances. made to perform a circuit of four miles; being conducted for two miles along wires supported on baked sticks, and for the remaining distance through the dry ground. As far as could be ascertained by the most careful observation, the time in which the discharge was transmitted along that immense circuit was perfectly instantaneous. A retardation in the passage of electricity, however, does take place, if the conductor be not of a sufficient size; and when this is the case, as well as in those instances where the conductor is not a good one, the discharge will not be effected so instantaneously or so completely. Under these circumstances, also, there is a tendency in the fluid to diverge from the direct line of its course, and to fly off to different objects in the vicinity, as is often exemplified in the case of lightning, which, on striking a building, is apt to take a very irregular and seemingly capricious route, darting towards conducting bodies which may happen to attract it, although at some distance from the immediate direction was pursuing. The motion of electricity through perfect conductors is attended with no perceptible alteration in the mechanical properties of the conducting

It was

bodies, provided they be of sufficient size for the charge of the electric fluid transmitted. On the contrary, very considerable effects are produced when a powerful charge is sent through a wire, which is too small to allow the whole quantity to pass with perfect freedom; or through an imperfect conductor, though of a large size, as is proved when a tree is struck by lightning. A piece of dry writing paper, as well as pieces of dry, porous wood, are easily torn in pieces by an electric charge. VIII. Electricity exerts a most extensive and important influence in effecting changes in the temperature and chemical composition of bodies. The ignition and fusion of metals by the electric discharge, are phenomena which have been long observed. Thus, by passing a strong charge through slender iron wires, or the finest flatted steel, called pendulum wire, they are ignited, and partly melted into globules, and at the same time partially oxidated. If a slip of gold or silver leaf be placed on white paper, and a strong shock passed through it, the metal will disappear with a bright flash, and the impulse with which its particles are driven against the paper will produce a permanent stain of a purple or gray color. The colors produced in this way have been applied to impress letters or ornamental devices on silk and on paper. For this purpose, the outline of the required figure should be 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. The heat evolved by electricity, like most other of its effects, is in proportion to the resistances opposed to its passage. A rod of wood, of considerable thickness, being made part of the circuit, has its temperature sensibly raised by a very few discharges. Most combustible bodies are capable of being inflamed by electricity. Thus alcohol, ether, camphor, powdered resin, phosphorus or gunpowder may be set on fire. And the sparks taken from a piece of ice are as capable of inflaming bodies as those from a piece of red-hot iron. The oxidation of metals, through which accumulated electricity has been passed, is rather to be

459

ascribed to the tendency which they are known to possess of combining with the oxygen of the atmosphere when heated, than to any peculiar agency of electricity. A reverse process, however, is found to attend electrical discharges through metallic oxides, extricating their oxygen, and restoring them to the metallic state. When a succession of electric discharges from a powerful electric machine are sent through water, a decomposition of that fluid takes place, and it is resolved into its two elements of oxygen and hydrogen, which immediately assume the gaseous form. When this experiment is conducted in a suitable apparatus, and a shock is transmitted through the mixed gases thus obtained, they are instantly kindled; a reunion of the elements takes place; and precisely the same quantity of water is reproduced as was decomposed to furnish the gases. It may appear somewhat paradoxical that the same agent should, in the course of the same experiment, produce at one time decomposition, and at another combination, of the same elements. The simplest way of reconciling this apparent discordance, is to suppose that the combination of the gases is the effect of the heat evolved during its forcible transit through an aeriform fluid that opposes considerable resistance to its passage; while the decomposition of the liquid is the direct consequence of the agency of electricity when not interfered with by heat. When a solution of sulphate of copper is subjected to the action of electricity by means of slender conducting wires terminating in the vessel containing the solution, the copper is revived, or precipitated in a metallic state, around the negative wire; but, upon reversing the direction of the current of electricity, so that the same wire now becomes positively electrified, the copper which has collected around it is redissolved, and a similar deposit takes place on the opposite wire, which now becomes the negative one. Similar experiments, made with other metallic solutions, are attended with similar results; and solutions of neutral salts with alkaline and earthy bases obey the same law, being separated into their constituent parts, the ingredient containing oxygen always appearing at the positive wire, and the base at the negative wire; but as these are a class of effects which have been more particularly investigated by that mode of agency denominated galvanism, we shall reserve a more full account of them for that article.

IX. Having seen the effects of electricity on inanimate matter, we now proceed to