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 these 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 causes of which the friction of surfaces is one), the state of union in which the two electricities exist in bodies, is disturbed: the vitreous electricity is imvelled 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, that is, 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 decompo-distance; that this fluid is dispersed through the sition attaches itself to the glass; the resinous to the amalgam. What remains in each surface undecomposed, continues to be quite inert. 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 to the results of this law, as declared 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 attraction, 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. 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 en ter 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. 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. 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. 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 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 oversaturated with this fluid; that of negative electricity, in a deficiency of fluid, or in matter under-saturated, 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 versá; 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, 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 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 re-render the arrangement of these parts more intelligimaining 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 of 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 clectrified, 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 regard to transference 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 hardly any fact that 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. When viewed as a mere hypothesis, calculated to facilitate our comprehension of the phenomena and of their connections, 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 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 ble, we will describe one of the simplest and best of the cylindric machines. The glass cylinder a b, Fig. 4, Plate I. Electricity, 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 c. Its pressure against the cylinder is regulated by an adjusting screw adapted to the wooden base at e, 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, d, which is sewed on the 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 or the cylinder, which is given by a single handle or by a multiplying wheel, 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 shall 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 readily 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 | trified, will exhibit the appearance of terror from the electrified, and light balls suspended by threads at f bristling up and divergence of the hair. being oppositely electrified will attract each other. 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 effect 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 neighbourhood, 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 communicates with the ground. The alternate motion of the ball between the two bells will keep up a continual ringing. This amusing experiment has been applied to give notice of changes taking place in the electrical state of the atmosphere. The mutual repulsion of bodies that are similarly electrified gives rise to many interesting experiments. A small figure in the shape of a human head, covered with hair, when placed upon the conductor and elec Advantage is taken of the repulsive property of electrified bodies for the construction of an instrument adapted to measure the intensity of the electricity they may contain. This instrument is called an electrometer. That invented by Henley consists of a slender rod of very light wood, serving as an index, terminated by a small pith ball, and suspended from the upper part of a stem of wood, which is fitted to a hole in the upper surface of the conductor. An ivory semicircle or quadrant is affixed to the stem, having its centre coinciding with the axis of motion of the rod, for the purpose of measuring the angle of deviation from the perpendicular, which the repulsion of the ball from the stem produces in the movable rod. The number of degrees which is described by the index affords some evidence of the quantity of electricity with which the apparatus is charged, though the instrument cannot be viewed as afford ing an exact mea- on the prime conductor of the machine, the other on a stand. The gold leaf electrometer of Bennet, or rather electro scope, is a very delicate instrument, and is much employed for detecting the presence of electricity. It consists of two narrow slips of gold leaf, suspended parallel to each other in a glass cylinder (which secures them from disturbance by the air), and attached to the end of a small metallic tube, terminating above cither in a flat surface of metal or a metallic ball. Two slips of tin-foil are pasted to the inside of the cylinder, on opposite sides, in a vertical position, and so placed as that the gold leaves may come in contact with these, when their mutual repulsion is sufficiently powerful to make them diverge to that extent. These slips of tin-foil terminate in the foot of the instrument, and thus are in communication with the earth. A very minute charge of electricity, communicated to the upper end of the tube, is immediately transmitted to the gold leaves, which are thus made to repel each other; but, if the repulsion is such as to make them strike against the tinfoil, their insulation ceases, and their electricity is carried off, and, becoming neutral, they resume their original position. We may here notice the electrometer invented by Coulomb, and called by him the torsion balance. It consists of a cylindrical glass jar, covered at the top by a circular glass plate, with a hole in its centre, through which descends, nearly to the bottom of the jar, a single fibre of the web of the silk-worm, with a needle of gum-lac or a piece of straw coated by sealing-wax, affixed to its lower extremity. The needle is terminated at one end by a small pith ball, and at the other by a disc of varnished paper, to serve as a counterpoise to the ball. The upper end of the silk fibre is attached to a kind of button, having a small index, and capable of |