LESSONS IN ELECTRICITY. 333 



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 CunaBus (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, at- 

 tracting 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 sim- 

 plest and cheapest means in making these experiments. 



As a thoughtful and reflective boy you cannot, I think, help won- 

 dering at the power which your thorough mastery of the principles 



