LECTURE XXXV.
The same Subject continued.
No electric can be excited without producing electric appearances in the body with which it is excited, provided that body be insulated; for this insulated rubber will attract light bodies, give sparks, and make a snapping noise, upon the approach of a conductor, as well as the excited electric.
If an insulated conductor be pointed, or if a pointed conductor, communicating with the earth, be held pretty near it, little or no electric appearance will be exhibited, only a light will appear at each of the points during the act of excitation, and a current of air will be sensible from off them both.
The effect of pointed bodies is best explained on the supposition of the electric matter in one body repelling that in another; and consequently the electricity belonging to a body with a large surface making a greater resistance to the entrance of foreign electricity than that belonging to a smaller.
These two electricities, viz. that of the excited electric, and that of the rubber, though similar to, are the reverse of, one another. A body attracted by the one will be repelled by the other, and they will attract, and in all respects act upon, one another more sensibly than upon other bodies; so that two pieces of glass or silk possessed of contrary electricities will cohere firmly together, and require a considerable force to separate them.
These two electricities having been first discovered by producing one of them from glass, and the other from amber, sealing-wax, sulphur, rosin, &c. first obtained the names of vitreous and resinous electricity; and it being afterwards imagined that one of them was a redundancy, and the other a deficiency, of a supposed electric fluid, the former has obtained the name of positive, and the latter that of negative, electricity; and these terms are now principally in use.
Positive and negative electricity may be distinguished from each other by the manner in which they appear at the points of bodies. From a pointed body electrified positively, there issues a stream of light, divided into denser streams, at the extremities; whereas, when the point is electrified negatively, the light is more minutely divided, and diffused equally. The former of these is called a brush, and the latter a star.