air on the adjacent sides will be increased by the mutual attraction of the particles of the electric fluid, and that on the further sides diminished; consequently that the force will urge the bodies towards one another, the motion in both cases corresponding to the forces producing it. An attempt has thus been made to attribute electrical attractions and repulsions to the mechanical pressure of the atmosphere; it is, however, more than doubtful whether these phenomena can be referred to that cause, but certain it is that, whatever the nature of these forces may be, they are not impeded in their action by the intervention of any substance whatever, provided it be not itself in an electric state.

A body charged with electricity, although perfectly insulated, so that all escape of electricity is precluded, tends to produce an electric state of the opposite kind in all bodies in its vicinity; positive electricity tends to produce negative electricity in a body near it, an vice versa, the effect being greater as the distance diminishes. This power which electricity possesses of causing an opposite electrical state in its vicinity is called induction. When a body charged with either species of electricity is presented to a neutral one, its tendency, in consequence of the law of induction, is to disturb the electrical condition of the neutral body. The electrified body induces electricity contrary to its own in the adjacent part of the neutral one, and therefore an electrical state 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, having now opposite electricities, will attract each other. The attraction between electrified and unelectrified substances is therefore merely a consequence of their altered state, resulting directly from the law of induction, and not an original law. The effects of induction depend upon the facility with which the equilibrium of the neutral state of a body can be overcome, a facility which is proportional to the conducting power of the body; consequently, the attraction exerted by an electrified substance upon another substance previously neutral will be much more energetic if the latter be a conductor than if it be a non-conductor.

The law of electrical attraction and repulsion has been determined by suspending a needle of gum lac horizontally by a silk fibre, the needle carrying at one end a piece of electrified gold-leaf. A globe charged with the same, or with the opposite kind of electricity, when presented to the goldleaf, will repel or attract it, and will therefore cause the needle to vibrate more or less rapidly according to the distance of the globe. A comparison of the number of oscillations performed in a given time at different distances, will determine the law of the variation of the electrical intensity, in the same manner that the force of gravitation is measured by the oscillations of the pendulum. Coulomb invented an instrument which balances the forces in question by the force of the torsion of a thread, which consequently measures their intensity. By this method he found that the intensity of the electrical attraction and repulsion varies inversely as the square of the distance. Since electricity can only be in equilibrio from the mutual repulsion of its particles, - which, according to these experiments, varies inversely as the square of the distance, - its distribution in different bodies depends upon the laws of mechanics, and therefore becomes a subject of analysis and calculation. The distribution of electricity has been so successfully determined by the analytical inves tigation of M. Poisson and Mr. Ivory, that all the computed phenomena have been confirmed by observation.

It is found by direct experiment that a metallic globe or cylinder contains the same quantity of electricity when hollow that it does when solid; therefore electricity is entirely confined to the surface of bodies, or if it does penetrate their substance, the depth is inappreciable; consequently the quantity bodies are capable of receiving does not follow the proportion of their bulk, but depends principally upon the extent of surface over which it is spread; so that the exterior may be positively or negatively electric while the interior is in a state of perfect neutrality.

Electricity of either kind may be accumulated to a great extent in insulated bodies, and as long as it is quiescent it occasions no sensible change in their properties, though it is spread over their surfaces in indefinitely thin layers. When restrained by the non-conducting power of the atmosphere, the tension or pressure exerted by the electric fluid against the air which opposes its escape is in the ratio compounded of the repulsive force of its own particles at the surface of the stratum of the fluid and of the thickness of that stratum; but as one of these elements is always proportional to the other, the total pressure on every point must be proportional to the square of the thickness. If this pressure be less than the coercive force of the air, the electricity is retained; but the instant it exceeds that force in any one point the electricity escapes, which it will do when the air is attenuated, or becomes saturated with moisture.

The power of retaining electricity depends also upon the shape of the body. It is most easily retained by a sphere, next to that by a spheriod, but it readily escapes from a point: and, on the contrary, a pointed object receives it with most facility. It appears from analysis that electricity, when in equilibrio, spreads itself in a thin stratum over the surface of a sphere, in consequence of the repulsion of its particles, which force is directed from the centre to the surface. In an oblong spheroid the intensity or thickness of the stratum of electricity at the extremities of the two axes is exactly in the proportion of the axes themselves; hence, when the ellipsoid is much elongated, the electricity becomes very feeble at the equator and powerful at the poles. A still greater difference in the intensities takes place in bodies of a cylindrical or prismatic form, and the more so in proportion as their length exceeds their breadth; therefore the electrical intensity is very powerful at a point, where nearly the whole electricity in the body will be concentrated.

A perfect conductor is not mechanically affected by the passage of electricity, if it be of sufficient size to carry off the whole; but it is shivered to pieces in an instant, if it be too small to carry off the charge; this also happens to a bad conductor. In that case the physical change is generally a separation of the particles, though it may occasionally be attributed to chemical action, or expansion from the heat evolved during the passage of the fluid; but all these effects are in proportion to the obstacles opposed to the freedom of its course. The heat produced by the electric shock is intense, fusing metals, and even volatilizing substances, though it is only accompanied by light when the fluid is obstructed in its passage. Electrical light is perfectly similar to solar light in its composition; it seems to arise from the condensation of the air, during the rapid motion of electricity, and varies both in intensity and colour with the density of the atmosphere. Electricity is occasionally produced by pressure and fracture; several crystalline substances also become electric when heated, especially tourmaline, one end of which acquires positive, and the other negative electricity, while the intermediate part is neutral; but when broken through the middle, each fragment is found to possess positive electricity at one end, and negative at the other, like the entire crystal. Electricity is evolved by bodies passing from a liquid to a solid state, also by the production and condensation of vapour, which is consequently a great source of atmospheric electricity.

The atmosphere, when clear, is almost always positively electric; its electricity is stronger in winter than in summer, during the day than in the night. The intensity increases for two or three hours from the time of sunrise, then decreases towards the middle of the day, and again augments as the sun declines, till about the time of sunset, after which it diminishes, and continues feeble during the night. Atmospheric electricity arises from an evolution of the electric fluid during the evaporation that is so abundant at the surface of the earth; and clouds probably owe their existence, or at least their form, to it, for they consist of hollow vesicles of vapour coated with electricity; as the electricity is either entirely positive or negative, the vesicles repel each other, which prevents them from uniting and falling down in rain. The friction of the surfaces of two strata of air moving in different directions, probably developes electricity; and if the strata be of different temperatures, a portion of the vapour they always contain will be deposited; the electricity evolved will be taken up by the vapour, and will cause it to assume the vesicular state constituting a cloud. A vast deal of electricity may be accumulated in this manner, which may either be positive or negative, and should two clouds charged with opposite kinds approach within a certain distance, the thickness of the coating of electricity will increase on the two sides of the clouds that are nearest to one another; and when the accumulation becomes so great as to overcome the coercive pressure of the atmosphere, a discharge takes place, which occasions a flash of lightning. The actual quantity of electricity in any one part of a cloud is extremely small; the intensity of the flash arises from the very great extent of surface occupied by the electricity, so that the clouds may be compared to enormous Leyden jars thinly coated with the electric fluid, which only acquires its intensity by its instantaneous condensation,

An interchange frequently takes place between the clouds and the earth, but on account of the extreme rapidity of lightning it is difficult to ascertain whether it goes from the clouds to the earth, or shoots upwards from the earth to the clouds, though there can be no doubt that it does both. M. Halvig measured the velocity of lightning by means of the camera lucida, and estimates that it is probably eight or ten miles in a second, or about forty times greater than that of sound; and M. Gay-Lussac has ascertained that a flash of lightning sometimes darts more than three miles at once in a straight line.

A person may be killed by lightning, although the explosion takes place at the distance of twenty miles, by what is called the back stroke. Suppose that the two extremities of a cloud highly charged with electricity hang down towards the earth, they will repel the electricity from the earth's sur