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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 investigations 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 surfaces of bodies, or, if it does penetrate their substance, the depth is inappreciable : conse

quently 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 spheroid, 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 ex

pansion 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 the 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 accumula.

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