you to thoroughly analyze and understand the action of the Leyden-jar. In charging the jar, the outer coating is connected with the earth and the inner coating with the electrical machine. Let the machine, as usual, be of glass yielding positive electricity. When it is worked the electricity poured into the jar acts inductively across the glass upon the outer coating; attracting its negative and repelling its positive to the earth. Two mutually attractive electric layers are thus in presence of each other, being separated merely by the glass. When the machine is in good order and the glass of the jar is thin, the attraction may be rendered strong enough to perforate the jar.
Franklin saw and announced with clearness the escape of the electricity from the outer coating of the jar. His statement is that, whatever be the quantity of the "electric fire" thrown into the jar, an equal quantity was dislodged from the outside. We have now to prove by actual experiment that this explanation is correct.
Place your Leyden-jar upon a table, and connect the outer coating with your electroscope. There is no divergence of the leaves when electricity is poured into the jar.
But here the outer coating is connected through the table with the earth. Let us cut off this communication by an insulator. Place the jar upon a board supported by warm tumblers, or upon a piece of vulcanized India-rubber cloth, and again connect the outer coating with the electroscope. The moment electricity is communicated to the knob of the jar the leaves of Dutch metal diverge. Detach the wire by your discharger and test the quality of the electricity—it is positive, as theory declares it must be.
Consider now the experiment of Kleist and Cunæus (Fig. 23). You will, I doubt not, penetrate its meaning. You will see that in their case the hand formed the outer coating of the jar. When electricity was communicated through the nail to the water within, that electricity acted across the glass inductively upon the hand, attracting the one fluid and repelling the other to the earth.
Again I say, prove all things; and what is here affirmed may be proved by the following beautiful and conclusive experiment: Stand on your board, insulated by its four tumblers; or upon a sheet of gutta-percha, or vulcanized India-rubber. Seize the old Ley den-phial with your left hand, and touch the electroscope with the right, or with a lath or a wire held in the right. When electricity is communicated to the nail, the leaves immediately diverge by the electricity driven from your left hand through your body to the electroscope.
Here the nail may be electrified either by connecting it with the prime conductor of the machine, or by simply rubbing it with an excited glass rod. Indeed, I should prefer your resorting to the simplest and cheapest means in making these experiments.
As a thoughtful and reflective boy you cannot, I think, help wondering at the power which your thorough mastery of the principles
