To apply this to the particular instance above recited, suppose the end of an in sulated conductor to be brought near the prime conductor in a positive state, the attractive power of the first mentioned conductor is greater than that of the prime conductor, yet, not being sufficient to draw sparks, at the given distance, the only effect it can produce is to make the electric matter accumulate, and become more dense in that part of the prime con. ductor, near which it is presented ; by which accumulation the rest of the prime conductor becomes less electrified, as ex perience testifies. This accumulated bo dy of electricity repels, and consequently rarifies the electric matter naturally con tained in that end of the conductor, which is presented to the prime conductor ; the rest of the fluid becomes more dense, and the other parts of the conductor which is presented, exhibit signs of electricity ; yet, as this conductor in the whole con tains no more than its natural quantity, if the electric state be taken off by drawing the spark, and it be afterwards removed from the vicinity of the prime conductor, it becomes negative throughout, by rea son of the loss of the spark. If a con. ductor be presented to the prime con ductor in a negative state, the effects are reversed, the attraction being strongest at the prime conductor, and the accumu lation being in the conductor which is presented, it exhibits a negative state, which, being destroyed, upon removal it becomes positive, by reason of the spark which was given to it when apparently negative.
These effects are more considerable the less the distance is between the two conductors ; and the intercedent electric body is peculiarly affected : the manner of which may be better understood by ob serving the phenomena of non-electrics, separa ed by electises which are less lia bie to allow the passing of the spark than the air is.
Upon an insulated horizontal plate of metal, lay a plate of glass, considerably larger, so that there may be a rim of three or four inches projecting beyond the me tal on every side. Upon the glass lay another plate of metal, of the same size as the former, so as precisely to cover it. Electrify the upper plate, and the lower will exhibit signs of electricity. Conti nue the electrization, and the lower plate will emit sparks to an uninsulated body for a time, and afterwards cease Sepa rate the plates from the glass without un insulating them, and the glass will appear to be possessed of the contrary electrici ties on the opposite sides. That side which communicated with the prime con ductor, during the electrizaaon, will have a like electricity, and the other the con trary. Take off the electricity of the plates of metal, and carefully replace the glass on the lower, without destroying the insulation, and also replace the upper plate with the same precaution. Then, with one end of an insulated wire, not pointed, but knobbed at the ends, touch one of the plates, and bring the other end near the other plate : the consequence will be, that a strong and loud spark will pass between it and the wire, the electri ty of the glass will be discharged, and the plates and the wire will exhibit few or no signs of electricity.
Au electric body, the surfaces of which are thus possessed of the contrary elec tricities, is said to be charged. The insu lation of the lower metallic plate, and of the discharging wire is not necessary, ex cept for he purpose of drawing inferen ces, respecting the manner of charging the electric plate. If the electricity of the prime conductor be strong, and the glass thick, the discharge will often be made by a spark from the one metallic plate to the other, over the surface of the glass which projects on every side ; but if the glass plate be thin, in which case, at an equal intensity, it admits of a much greater charge, the discharge will be made through its substance. Glass, as thick as one eighth of an inch, may be pene trated by this means, one or more holes being made where the electric matter has passed, in which holes the glass is pulverised, and may be picked out with a pin.
It is not possible to charge an electric plate by inducing an electric state on one of its surfaces, unless the other be at the same time sufficiently near to a non-elec tric to assume the contrary state, by emit ting or receiving the electiic matter.
If a plate of glass be laid upon an un insulated plate of metal, the upper sur face may be rendered electric by friction, or by applying an electrified body succes sively to its parts. This electricity may be taken off by touching the upper sur face with an uninsulated metallic plate of the same dimensions as that upon which the glass is placed, but will not be entire ly taken off; because the communication between the two surfaces in this method is not perfect, and because the metal can not by ordinary means be brought into actual contact with the glass. The small quantity which remains, produces an ef fect which has been mistaken for a per petual electricity. For if a plate of me. tal, to which a glass handle is affixed, be laid upon the glass, this small quantity of electricity will influence the metal, and, without actually communicating the elec tric matter, will cause it to exhibit a simi lar state. If this be taken off, by drawing the spark, and the metal then removed, by 'Beans of the glass handle, it will be found possessed of the contrary state of electricity, and another spark may be ob tained. The metallic plate may be then again applied to the surface of the glass, and the process again repeated, and so on for a prodigious number of times, with out any sensible difference in the event. For the electricity at the surface of the glass being almost in the natural state, as to condensation, does not disappear for a very long time, and the very near ap proach of the metal enables it to produce the same effect as would be obtainad at a greater distance from a stronger electri city. This is made obvious, by bringing the metallic plate near the surface of the glass before its first strong electricity is taken oft; for the same event is then per ceived at the distance of four, five, or six inches, as in the former case is produced by contact.