results it appears that the inductive capacities of the dielectric bodies tried, that of air being expressed by unity, are proportional to the following numbers: These results agree very closely with those of the preceding table. § 339. During the experiments of Dr. Faraday upon the different inductive capacities of different bodies, the curious fact was established, that an actual penetration of the charge to some distance within the dielectric, took place at each of its two surfaces, by what is called conduction; so that the electric forces sustaining the induction, as has just been stated, are not upon the metallic surfaces only, (§ 337), but upon and within the dielectric also. Let a plate of any dielectric be armed in the usual way with metallic coatings, and a vitreous charge be communicated to one, while the other is uninsulated, and let the whole remain from ten to fifteen minutes. If then the coatings be discharged, insulated and immediately examined, no electricity will be found in them; but in a short time, upon a second examination, they will appear charged in the same way, though not to the same degree, as they were at first. A portion of the vitreous force, under the coercing influence of all the forces concerned, penetrates the dielectric, and takes up its place a little within its surface; a corresponding portion of the resinous force also assuming a similar position on the opposite surface. The discharge destroys or neutralizes all external induction, and the coatings are therefore found unelectrified; but it also removes almost the whole of the forces by which the electric charge was driven into the dielectric, and it therefore returns on its course to the surfaces, and constitutes the re-charge observed. Two plates of spermaceti (a substance whose insulating powers are not very perfect) were put together so as to form one plate, and coated with metal in the usual way. The system was charged, then discharged, insulated, examined, and found to give no indications of electricity. The plates were then separated from each other, and one was found in the vitreous, and the other in the resinous state, nearly all the electricities being in the metallic linings. Hence, it is clear that, of the forces sought for, the vitreous was in one half of the compound plate, and the resinous in the other half; for, when removed bodily with their plates from each other's inductive influence, they appeared in separate places, and resumed of necessity their power of acting by induction on the electricity of surrounding bodies. Had the effect depended upon a peculiar relation of the contiguous particles of matter only, each half plate should have shown vitreous force on one surface, and resinous on the other. It is the assumption, for a time, of this charged state of the glass, between the coatings of the Leyden jar, which gives origin to the phenomenon, which is called the residual charge. After a large battery has been charged for some time, and then discharged, it is found that it will spontaneously recover its charge to a very considerable extent; and this is due to the return of the electricity in the manner just described. § 340. Opposed to the phenomena of the charge which we have now examined, are those of the discharge of the electric forces, which yet remain for our closer investigation, There are various modes by which this may be effected: the discharge by conduction, by disruption, and by convection, come before us on the present occasion. $341. 1st. The discharge by conduction involves no chemical action, or displacement of particles. Insulation and conduction might at first be supposed to be directly opposed properties of matter, but Dr. Faraday has shown that they are only extreme degrees of one common condition, and that they are connected by numerous intermediate links. At the two extremes stand the gases and the metals; and spermaceti is an example of a substance through which induction can take place, and also conduction; but both in a reduced degree. A certain condition of particles which, if retained, constitutes insulation, if lowered by the communication of forces from one to the other, constitutes conduction. In the state of pola rity or tension, the particles of all bodies (with the exception possibly of the gases) have a capability of communicating their forces in various times one to the other, by which they are lowered, and discharge ultimately ensues. Shell-lac is the best insulator known amongst solids; that is to say, it retains its state of polarity most perfectly, but it becomes lowered in time, and is also capable of having that forced charge communicated to it which we have just described, and which is equivalent to conduction. The metals themselves, although such excellent conductors as to admit of the passage of electricity through them with a velocity which baffles the powers of conception, offer different degrees of resistance to its transfer: and such resistance is pro tanto insulating power. § 342. Mr. Snow Harris has contrived a beautiful apparatus for comparing this resistance of the metals. It consists of an air thermometer, through the bulb of which a wire of the metal to be examined passes. By the passage of a definite portion of electricity from a Leyden jar through this wire, it becomes heated in proportion to its resistance, and this heat acting upon the air causes an expansion, which is measured by the liquid in the stem. The results of some of his experiments are comprised in the following table: The ready communication of forces between contiguous particles constitutes conduction; and the difficul tcommunication, insulation. Conductors and insulators are bodies whose particles naturally possess the property of communicating their respective forces easily, or with difficulty: the latter requiring the polar forces to be raised to a higher degree than the former, before this transference or communication can take place. §343. The resistance of the metals, or their insulating power, may be brought into direct comparison with air, and even made to exceed it, by a very simple contrivance. Let one end of a very long wire be placed in connexion with the outside coating of a charged Leyden jar, and let it be doubled in such a way as to allow the other extremity to be brought very near, but not to touch, the same coating with it. Let a connexion now be made by a discharging rod between the inside coating and this extremity of the wire, and if the distance be properly adjusted, a portion of the charge will pass in a spark through the interval of air between the two extremities, rather than traverse the whole length of the wire. The middle part of the wire, therefore, acts as an insulating medium, and the tension which causes the spark at the extremities must exist throughout its length. § 344. The charge, as it passes through a metallic wire, momentary as is its duration, acts by induction through the air towards surrounding objects; and if the knob of a small Leyden jar be presented to such a conductor, a lateral spark will pass to it. But the arrangement which exhibits this effect to the greatest advantage, is that of a long fine copper wire, insulated parallel to the horizon, and terminated at each end by a small ball. When sparks are thrown upon this from a globe of about a foot in diameter, the wire at each discharge becomes beautifully luminous from one end to the other, even if it be a hundred feet long: rays are given off on all sides perpendicular to the axis of the wire. When the long wire is arranged in two parallel, but continuous lines, by bending it, the outer side only of each arm becomes luminous. When formed into three parallel lines by a double bend, the middle portion of the wire does not become luminous, the outer sides only of the outer lines of wire exhibit the rays. When the wire is formed into a flat spiral, the outer spiral alone exhibits the lateral discharge; but the light in this case is very brilliant; the inner spirals appearing to increase the effect. The inner surfaces of the wires, in these latter cases, being all turned towards similarlyelectrified surfaces, the lateral process of induction cannot of course take place. § 345. It may be stated generally, that whenever two or more passages are open, by which the electric discharge may |