When, by any operation of art or nature, there happens to be a greater proportion of this fluid in one body than in another, the body which has most will communicate to that which has least, till the proportion becomes equal, provided the distance between them be not too great; or, if it be too great, till there be proper conductors to convey it from one to the other.
If the communication be through the air, without any conductor, a bright light is seen between the bodies, and a sound is heard. In small experiments, we call this light and sound the electric spark and snap; but in the great operations of nature, the light is what we call lightning, and the sound (produced at the same time, though generally arriving later at our ears than the light does in our eyes) is, with its echoes, called thunder.
If the communication of this fluid be by a conductor, it may be without either light or sound, the subtle fluid passing in the substance of the conductor.
If the conductor be good, and of sufficient bigness, the fluid passes through it without hurting it. If otherwise, it is damaged or destroyed.
All metals, and water, are good conductors. Other bodies may become conductors by having some quantity of water in them, as wood and other materials used in building, but not having much water in them, are not good conductors, and therefore are often damaged in the operation.
Glass, wax, silk, wool, hair, feathers, and even wood perfectly dry, are non-conductors: that is, they resist instead of facilitating the passage of this subtle fluid.
When this fluid has an opportunity of passing through two conductors, one good and sufficient, as of metal, the other not so good, it passes in the best, and will follow in any direction.
The distance at which a body charged with this fluid will discharge itself suddenly, striking through the air into another body that is not charged, or not so highly charged, is different according to the quantity of the fluid, the dimensions and form of the bodies themselves, and the state of the air between them. This distance, whatever it happens to be between any two bodies, is called their striking distance, as, till they come within that distance of each other, no stroke will be made.
The clouds have often more of this fluid in proportion than the earth: in which case, as soon as they come near enough, (that is, within the striking distance,) or meet with a conductor, the fluid quits them and strikes into the earth. A cloud fully charged with this fluid, if so high as to be beyond the striking distance from the earth, passes quietly without making noise or giving light, unless it meet with other clouds that have less.
Tall trees and lofty buildings, as the towers and spires of churches, become sometimes conductors between the clouds and the earth; but, not being good ones, that is, not conveying the fluid freely, they are often damaged.