When two phials were both charged through their knobs, a cork ball, suspended between them by a silken thread, was first attracted and then repelled by both the knobs as they were positively electrified. When both the phials were charged through their outside coat ings, the ball was also i•._pelled by them both ; but when one of the phials was charged by its knob, and the other by its outside coating, the suspended cork ball played vigorously between them both, till the jars were nearly discharged. Hence it follows, that the one side of the phial possesses always the opposite elec tricity to the other, and that the two sides are simul taneously electrified, whether the electricity is drawn from the prime conductor by the one side or the other.
As glass is impenetrable to the electric fluid, it neces sarily followed that the electricity was not communica ted from the one side of the glass to the other ; and this was easily ascertained by observing, that when the jar was insulated it was not capable of being charged but in a very trifling degree. Hence it followed, that the electricity of the outside of the jar was supplied from the earth by the conducting bodies which surrounded it, and that whenever one side of an electric receives one kind of electricity, the opposite side is simultane ously supplied with the opposite electricity, either from the earth or other conducting bodies which are near it.
The next object of Dr Franklin was, to ascertain if the positive electricity of one side of the phial was equal to the negative electricity of the other. For this purpose lie hung a small linen thread near the outside coating of a charged phial, and every time that he presented his finger to the wire, the thread At as attract ed by the coating, the electricity taken from the inside by touching the wire being exactly equal to what was drawn in on the outside by the thread. Professor Rich man obtained the same result by a very elegant expe riment. Having coated both surfaces of a pane of glass within two inches of its edge, he fixed linen threads to the upper part the coating on both sides. When the plate was not charged, the linen threads hung down in contact with the coating ; hut when the plate was placed in a vertical position and then char ged, the threads were repelled from the coating, and formed equal angles with it on both sides. When his finger or any conductor was brought near one side, the thread on the same side formed a less angle with the coating, while the thread on the opposite side form ed an angle as much greater. Upon bringing his finger
in contact with one of the sides, the corresponding thread fell down upon the coating, while the opposite thread formed twice the angle with the coating which it had clone before. The angle formed by the two threads with each other was therefore always a constant quantity.
Dr Franklin at first imagined that the electricity accu mulated in the jar existed in the coatings in contact with the glass, but he soon found upon farther examination, that the electricity 'vas contained in the glass itself, and that the sole use of the coating was, in the act of charging the jar, to conduct the electricity to all the parts of the glass which was in contact, and in the act of discharging it, to facilitate the communication be tween the two sides of the glass. In order to ascertain these points, he charged a jar, and placed it upon glass, or any insulating stand. He then took out the cork and the wire, ,and ascertained that they did not contain any electricity, by touching the outside coating ttith one hand, and putting the finger of the other hand into the mouth of the bottle, when lie received a shock as powerful as if the wire and cork had been in their place. Having put water into the phial, which bcin,g a con ductor, has always the same effect as a tinfoil coating, he charged the jar, and then pouring the water into an insulated bottle, found that it also would not give the shock. It then struck him, that the electricity must either have gone of in the act of pouring out the wa ter, or must still be resident in the bottle. Ile there fore filled the phial with fresh water, and without any new charge he received the shock as before, which completely proved that the electricity resided in the glass. Dr Franklin made similar experiments with coated panes of glass, and obtained the very same re sult, by changing the coating as he had changed the water. The same truth may perhaps be more satisfac torily established by the following experiment. Fix the outside coating of an uncoated jar with a little grease ; and, instead of the inside coating, substitute a quantity of mercury, or of small shot, or gold leaf, or brass filings. As soon as it is charged, turn it upside down, so as to allow the mercury or shot to fall out along with the wire and knob, and remove also the outside coating. By this operation, the jar will not have lost its charge ; for if new mercury or shot be put into the inside, and a new piece of tinfoil on the outside, the shock will be felt as strongly as if no charge had taken place.