cumstances, are, 1. the imperfection of the insulating property in the solids by which they are supported; 2. the contact of successive portions of air, every particle of which carries off a certain quanti ty of electricity; 3. the deposition of moisture upon the surface of the insulating bodies, which establishes communications between their opposite ends, and may be considered as virtually increasing their conducting power. Still another circumstance, which materially affects the dissipation of electricity, is the shape of the body in which it is accumulated. The form most favorable for its retention is that of a sphere; next, a cylinder terminated at both extremities by a hemisphere. On the other hand, electricity escapes most readily from bodies of a pointed figure, especially if the point projects to a distance from the surface. In such bodies, it is scarcely possible to retain any accumulation of the electric fluid; whereas, pointed bodies receive electricity more readily than those of any other form.Electric excitation in different bodies exhibits different phenomena. We have seen that light substances excited by glass repel one another, and are likewise repelled by the excited glass. The same thing also happens with respect to bodies which have received their electricity from excited sulphur, or sealing-wax. But on examining the action of any of the bodies of the former class upon any of those belonging to the latter, we find that, instead of repelling, they attract each other; and what is still more remarkable, the instant these bodies come in contact, provided they have both been electrified in an equal degree, they cease at once to exhibit any signs of electrical excitement; the electricity in the one appearing to neutralize that in the other. Thus we seem to have evidence of two kinds of electricity; and as these were first noticed, the one in glass and the other in resinous bodies, they were named vitreous and resinous electricity. Their mode of action on matter has been expressed by the following general law, viz.: Bodies charged with either species of electricity, repel bodies charged with the same species, but attract bodies charged with the other species; and at equal distances, the attractive power in the one case is exactly equal to the repulsive power in the other. Accordingly, we learn the kind of electricity with which a given body is charged, by approaching it to an insulated pith ball, which has previously been touched either with excited glass, or with excited sealing-wax. It is known,

moreover, that, when two electrics are rubbed against one another, the one acquires, always, one kind of electricity, the other the opposite; and both are produced in equal degrees. Thus, when glass is rubbed by silk or flannel, just as much resinous electricity is produced in the silk or flannel, as there is vitreous electricity produced in the glass; and, consequently, as they are endowed with opposite electricities, there should be an attraction existing between the excited surfaces of the bodies rubbed. This fact is easily proved by the simple and familiar experiment of the ribbons. If a white and a black ribbon, of two or three feet long, and perfectly dry, be applied to each other by their smooth surfaces, and are then drawn repeatedly between the finger and thumb, so as to rub against each other, they will be found to adhere together, and, if pulled asunder at one end, will rush together with great quickness; while united, they exhibit no sign of electricity, because the operation of the one is just the reverse of that of the other, and their power is neutralized and inoperative. If completely separated, however, each will manifest a strong electrical power, the one attracting those bodies which the other repels. The causes that determine the species of electricity excited in the respective bodies, of which the surfaces are made to rub against each other, have not been satisfactorily ascertained. The mechanical configuration of the surfaces appears to have more influence in the result, than the nature of the substances themselves. Thus smooth glass acquires vitreous electricity by friction with almost every substance, except the back of a cat, which induces the resinous electricity; but roughened glass, if rubbed with the same substances, becomes charged with resinous electricity, while the rubbing bodies acquire the vitreous. Silk, rubbed by resin, takes the vitreous, but with polished glass, the resinous electricity. The following is a list of several substances, which acquire vitreous electricity, when rubbed with any of those which follow it, in the order in which they are set down; and resinous electricity, if rubbed with any of those which precede :— The back of a cat. Paper. Polished glass. Silk. Woollen cloth. Gum-lac. Feathers. Roughened glass. Wood.

In the experiment above mentioned of the silk ribbons, the black ribbon exhibited the vitreous, and the white one the res

inous electricity. But when the ribbons are differently excited, as the one being drawn lengthwise and at right angles over a part of the other, the one which has suffered friction in its whole length acquires vitreous, and the other resinous electricity. Indeed, the slightest difference in the conditions of these and similar experiments, or the species of electricity arising from friction, will be often sufficient to produce opposite results. Another important observation, with regard to electrical phenomena, requires to be stated previous to our conclusion of the present head. Whenever a body is charged with electricity, although it be perfectly insulated, it tends to produce an opposite electrical state in all the bodies in its vicinity, and this with greater energy in proportion as the distance is smaller. This effect is termed the induction of electricity. In consequence of this law, if an electrified body, charged with either species of electricity, be presented to an unelectrified or neutral body, the electrical condition of the different parts of the neutral body is disturbed. The electrified body induces a state of electricity contrary to its own, in that part of the neutral body which is nearest to it, and consequently a state of electricity similar to its own in the remote part. Hence the neutrality of the second body is destroyed by the action of the first; and the adjacent parts of the two bodies, having now opposite electricities, will attract each other. It thus appears, that the attraction which is observed to take place between electrified bodies and those that are unelectrified, is merely a consequence of the altered state of those bodies, resulting directly from the law of induction.

II. The hypothesis which naturally suggests itself for the explanation of the phenomena above stated, is that of a very subtile, imponderable and highly elastic fluid, pervading all material bodies, and capable of moving with various degrees of facility through the pores or actual substance of different kinds of matter. In some, as in those we call conductors or non-electrics, it moves without any apparent obstruction; while in others, as in those we call non-conductors or electrics, it moves with difficulty. Moreover, as the phenomena appear to indicate the agency of two kinds of fluid, we shall, for the present, assume the existence of two species, and shall speak of these under the names of the vitreous and the resinous electricities. They must each have, when separate, the same general properties as have already been enumerated above;

while, in relation to each other, there must be a complete contrariety in their nature, so that, when combined together, their action on the bodies in their immediate vicinity shall cease. And it is when existing in this state of union or neutrality, that bodies are said to be in their natural state as respects electricity. We shall now proceed to compare the suppositions we have made with the facts, as presented to us by nature, and developed by experi ment.-a. Facts connected with excitation. From various causes (of which the friction of surfaces is one), the state of union in which the two electricities naturally exist in bodies is disturbed: the vitreous electricity is impelled in one direction, while the resinous is transferred to the opposite; and each manifests its peculiar powers. When accumulated in any body, each fluid acts in proportion to its relative quantity, i. e., to the quantity which is in excess above that which is still retained, in a state of inactivity, by its union with electricity of the opposite kind. Thus, when glass is rubbed with a metallic amalgam, a portion only of the electricities at the two surfaces is decomposed: the vitreous electricity resulting from this decomposition attaches itself to the glass; the resinous to the amalgam. What remains in each surface undecomposed, continues to be quite inert.-b. Facts connected with distribution. Both of these fluids, being highly elastic, their particles repel one another with a force which increases in proportion as their distance is less; and this force acts at all distances, and is not impeded by the interposition of bodies of any kind, provided they are not themselves in an active electrical state. It has been deduced, from the most careful analysis, that this force follows the same law with that of gravitation: viz. that its intensity is inversely as the square of the distance. The mode in which the electricity imparted to a conducting body, or to a system of conductors, is distributed among their different parts, is in exact conformity with the results of this law, as deduced by mathematical investigation. While the particles of each fluid repel those of the same kind, they exert an equally strong attraction for the particles of the other species of electric fluid. This attrac tion, in like manner, increases with a diminution of distance, and follows the same law as to its intensity: viz. that of the inverse ratio of the square of the distance. This force, also, is not affected by the presence of any intervening body.-c. Facts connected with transferrence. Since the

two electricities have this powerful attraction for each other, they would always flow towards one another, and coalesce, were it not for the obstacles thrown in their way by non-conductors. When, instead of these, conducting substances are interposed, they enter into union with great velocity, producing, in their transit and confluence, several remarkable effects. When once united, their powers remain dormant, until again called into action by the renewed separation of the fluids.-d. Facts relating to attraction and repulsion. The repulsion which is observed to take place between bodies that are insulated, and charged with any one species of electricity, and other bodies similarly charged, is derived from the repulsive power which the particles of this fluid exert towards those of their own species; and the attractions between bodies differently electrified, is derived from the attractive power of the vitreous particles for those of the opposite kind. In all cases, the movements of electrified bodies represent the forces themselves which actuate the particles of the developed electricities they contain.-e. Facts relating to induction. Wherever one of the electricities exists in an active state, it must repel all the particles of the same electricity in all surrounding bodies, and attract those of the opposite species. Thus the law of induction is seen to be a direct consequence of the hypothesis we are considering. Thus far we have proceeded upon the hypothesis of two distinct electric fluids. It was, however, discovered by Franklin, that it is equally easy to account for all the phenomena, on the supposition of their resulting from the agency of a single electric fluid. This theory supposes, that the single agent in question, and which we shall call the electric fluid, is highly elastic or repulsive of its own particles, the repulsion taking place with a force varying inversely as the square of the distance; that its particles attract and are attracted by the particles of all other matter, following the same law of the inverse square of the distance; that this fluid is dispersed through the pores of bodies, and moves through them with various degrees of facility, according as they are conductors or non-conductors. Bodies are said to be in their natural state, with regard to this fluid, when the repulsion of the fluid they contain of a particle of fluid at a distance, is exactly balanced by the attraction of the matter in the body for the same particle; and, under these circumstances, they exhibit no electrical phenomena.

But if subjected to certain operations, as friction, the equilibrium is destroyed, and they acquire more or less than when in their natural state. Whenever they acquire a quantity of fluid greater than in their natural state, they are said to be positively electrified, or to be electrified plus, and present the phenomena ascribed to what was called vitreous electricity.When, on the other hand, there is a quantity less than what is required in order to be in their natural state, they are said to be negatively electrified, or to be electrified minus; in which case they correspond with the state of resinous electricity. The state of positive electricity, then, consists in a redundance of the electric fluid, or in matter over-saturated with this fluid; that of negative electricity, in a deficiency of fluid, or in matter undersaturated, or, what may be considered the same thing, in redundant matter. In considering the mutual electrical actions of bodies, the portions in which the matter and the fluid mutually saturate each other, need not be taken into account, since their actions, as we have seen, are perfectly neutralized; and we need only attend to those of the redundant fluid and the redundant matter. When a body contains more than its natural proportion of electric fluid, the surplus will, by the repulsive tendency of its particles, overflow and escape, unless prevented by insulation, until the body is reduced to its neutral state. When under-saturated, the redundant matter will attract fluid from all quarters, from which it can receive, until it is again brought to its natural state. The mutual recession of two positively electrified bodies is a direct consequence of the redundance of the electric fluid contained in each, this fluid being attracted to the matter by its attraction for it in both bodies; and the fluid in one being repulsive of the fluid in the other, the bodies are necessarily impelled in the direction of the repulsion. In the same manner, the mutual attraction between two bodies, one of which is electrified plus, and the other minus, is the immediate effect of the attraction of the redundant fluid in one for the redundant matter in the other, and vice versa; for this attraction is mutual. The mutual recession of two bodies, negatively electrified, does not appear to be accounted for upon the Franklinian theory. In order to do this, therefore, it has been found necessary to append to it the following provision: that particles of simple matter, or bodies unsaturated with the electric fluid, are mutually

repulsive. Without this provision, indeed, we are unable to explain the want of action between two neutral bodies; for, the repulsion of the fluids in both bodies being balanced by the attraction of the fluid in the one for the matter in the other, the remaining attraction of the fluid in the second body for the matter in the first, would be uncompensated by any repulsion; and the forces would not be held in equilibrium, as we find they really are. The law of electrical induction is an immediate consequence of the Franklinian theory. When a body charged with electricity is presented to a neutral body, the redundant fluid of the former exerts a repulsive action on the fluid in the latter body; and if this happens to be a conductor, it impels a certain portion of that fluid to the remote end of this body, which becomes at that part positively electrified; while its nearer end, which the same fluid has quitted, is consequently in the state of negative electricity. If the first body had been negatively electrified, its unsaturated matter would have exerted an attractive force on the fluid in the second body, and would have drawn it nearer to itself, producing an accumulation or redundance of fluid at the adjacent end, and a corresponding deficiency at the remote end; that is, the former would have been rendered positive, and the latter negative. All this is exactly conformable to observation. The facts with respect to transferrence are easily explicable upon this hypothesis, and they arise from the destruction of the equilibrium of forces, which confined the fluid to a particular situation or mode of distribution. Indeed, there is no fact which is explained on the hypothesis of two fluids, which is not equally explicable on the Franklinian theory; and the explanations by the first are easily converted into those of the second by substituting the expressions of positive and negative for those of vitreous and resinous electricities. The principal advantage of Franklin's system is, its superior simplicity. On the other hand, the phenomena of galvanism prove that the two electricities, whatever may be their nature, exert very different chemical agencies, and hence, whichever theory we may choose to adopt, it is necessary, in their chemical history, always to preserve the distinction between them. When viewed, however, as a mere hypothesis, calculated to facilitate our comprehension of the phenomena and of their connexions, it is a matter of indifference which we employ, since they will either of them answer the purpose. For the future, however, we shall more

generally employ the language of the Franklinian theory, on account of its greater convenience.

III. Electrical Machines. The essential parts of an instrument for procuring large supplies of electricity for the purposes of experiment, are the electric, the rubber, the prime conductor, the insulator, and the machinery for setting the electric in motion. The electric, by the excitation of which the electricity is to be developed, may be made of various substances. Polished glass has, however, received the preference. Its form is that of a hollow cylinder, or of a flat circular plate, revolving upon a horizontal axis. The cushion is usually made of soft leather, generally basil skin, stuffed with hair or wool, so as to be as hard as the bottom of a chair, but yet sufficiently yielding to accommodate itself, without much pressure, to the surface of the glass to which it is applied. The prime conductor is a cylindrical tube, each end terminating in a hemisphere. There is no advantage in its being made solid, for the electricity is only contained at the surfaces. It may be made of thin sheet brass or copper, or tin, or of pasteboard covered with gold leaf or tin foil. Care must be taken that its surface be free from all points and asperities; and the perforations which are made in it, and which should be about the size of a quill, for the purpose of attaching wires and other kinds of fixtures, should have their edges well rounded and smoothed off. In order to render the arrangement of these parts more intelligible, we will describe one of the simplest and best of the cylindric machines. The glass cylinder is from 8 to 16 inches in diameter, and from 1 to 2 feet long, supported, for the purpose of insulation, on two upright pillars of glass, which are fixed to a firm wooden stand. Two hollow metallic conductors, equal in length to the cylinder, and about one fourth of its diameter, are placed parallel to it, one on each side, upon two insulating pillars of glass, which are cemented into two separate pieces of wood, that slide across the base so as to allow of their being brought within different distances from the cylinder. To one of these conductors the cushion is attached, which is of the same length with the conductor. Its pressure against the cylinder is regulated by an adjusting screw adapted to the wooden base, on which the glass pillar that supports the conductor is fixed. From the upper edge of the cushion there proceeds a flap of thin oiled silk, which is sewed on the

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cushion about a quarter of an inch from its upper edge. It extends over the upper surface of the glass cylinder to within an inch of a row of metallic points, proceeding, like the teeth of a rake, from a horizontal rod, which is fixed to the adjacent side of the opposite conductor. The motion of the cylinder, which is given by a single handle or by a multiplying wheel, 1 must always be given in the direction of the silk flap. That part of the cushion which comes in contact with the glass cylinder, should be coated with an amalgam of tin, zinc and mercury, applied by means of hog's lard. The amalgam should be placed uniformly over the cushion, until level with the line formed by the seam which joins the silk flap to the face of the cushion. No amalgam should be placed over this line, nor on the silk flap; and it is even requisite to wipe the silk flap clean whenever the continued motion of the machine should have soiled it by depositing dust or amalgam on its surface. The best amalgam is formed by melting together one ounce of tin and two ounces of zinc, which are to be mixed, while fluid, with six ounces of mercury, and agitated in an iron or thick wooden box until cold. It is then to be reduced to very fine powder in a mortar, and mixed with a sufficient quantity of hog's lard to form it into a paste. The mode in which the electrical machine just described acts, will read ily be understood. The friction of the cushion against the glass cylinder produces a transfer of electric fluid from the former to the latter; that is, the cushion becomes negatively and the glass positively electrified. The fluid, which thus adheres to the glass, is carried round by the revolution of the cylinder, and its escape is at first prevented by the silk flap which covers the cylinder, until it comes to the immediate vicinity of the metallic points, which, being placed at a small distance from the cylinder, absorb nearly the whole of the electricity as it passes near them, and transfers it to the prime conductor. Positive electricity is thus accumulated in the prime conductor, while the conductor connected with the cushion, being deprived of this electricity, is negatively electrified. If both these conductors are insulated, this action will soon have reached its limit; for when the cushion and its conductor have been exhausted of their fluid to a certain degree, they cannot, by the same force of excitation, supply any further quantity to the glass. In order to enable it to do so, we must replenish it, or restore to it a quantity equal to what it

has lost. This is done by destroying the insulation of the cushion through the means of a metallic chain or wire, extending from it to the earth, which is the great reservoir of the electric fluid. The prime conductor will now be supplied with a constant stream of positive electricity. If it be our object, on the other hand, to accumulate negative electricity by the same instrument, we have only to insulate the conductor to which the cushion is attached, and to connect the prime conductor with the ground, in order to allow the fluid to escape from it as soon as it is collected from the cylinder. The fluid will thus continue to be drawn, without interruption, from the negative conductor, as it now meets with no impediment to its discharge on the opposite side of the machine. That the quantity of positive electricity produced in one conductor is exactly equal to that of the negative electricity in the other, is proved by the fact, that, if the two conductors are connected by a wire, no signs of electricity are obtained in any of the conductors on turning the machine. A person standing on a stool with glass legs is thereby insulated; and if, in this situation, he touch the prime conductor, either with his hand or through the medium of a metallic rod or chain, he may be considered as forming part of the same system of conductors. When the machine is worked, therefore, he will partake, with the conductor, of its charge of electricity, and sparks may be drawn from any part of his body by the knuckle of any other person who is in communication with the ground.

IV. The effects of electrical attraction and repulsion may now be exhibited much more distinctly with the aid of those considerable accumulations of electricity which we are enabled to form by the electrical machine. A pith ball, or a fragment of gold leaf, is very strongly and immediately attracted by the electrified conductor; and the instant after it has come into contact with it, it is repelled; but it is now attracted by the other bodies in its neighborhood, to which it communicates its own electricity, and then is again in a state to be influenced by the conductor, and to be again attracted; and this alternation of effects will continue as long as the conductor remains charged. This alternation of attractions and repulsions accompanying the transferring electricity by movable conductors, is also illustrated by the motions of a ball suspended by a silk thread, and placed between two bells, of which the one is electrified, and the other