Leyden jar
Franklin explained in these letters that in an experiment he did show that the charge was in the glass itself and not in the water within the jar, as had been theorized by others. After a Leyden jar was charged he poured the water from it into another Leyden jar that was not charged. It turned out the electric force was not "condensed" in the water itself, as the new Leyden jar had no charge with this water from the first Leyden jar. Then the first jar was refilled with fresh water and it was discovered that there was a charge in this jar. The conclusion then was that it was the glass itself that was the condenser of the electric force. It was further determined by more experimentation that it did not matter what shape the glass was in or if an object was shaped like a glass bottle. It was finally determined that a property of the glass itself was this "force" of equal and opposite charges.