Water-filled Leyden jar — Drawing of an early form of Leyden jar, from an 1898 textbook on physics. Unlike the later type of Leyden jar which had coatings of metal foil on the inside and outside, this first form of Leyden jar was filled with water; the water formed the inner plate of the capacitor. A metal nail driven through the cork stopper made contact with the water, allowing the water to be charged with electricity and discharged. The jar was held in the hand, and the (grounded) hand on the outside of the jar formed the other plate of the capacitor. Once charged, the jar could be discharged by approaching the nail with a finger as shown. The charge from the water would jump via a spark to the hand, and flow through the body to the other hand holding the jar, neutralizing the opposite charge there. This often resulted in a nasty shock.
Photo Credit: By W. Jerome Harrison - Public Domain, https://commons.wikimedia.org/w/index.php?curid=10942385