132 



ELECTRICITY. 



" When the tube is again presented to the opposite end of the conductor, the 

 part of the natural electricity which the threads had lost is again restored to 

 them by the repulsion of the tube forcing the electric fluid toward them from 

 other parts of the conductor, and thus restoring them to their natural state. 

 When the tube is once more withdrawn, the fluid is again equally diffused, and 

 tho threads, as before, are negatively electrified. 



" Finally, when the tube is presented to the threads already diverging with 

 negative electricity, still more of their natural electricity is repelled by the ex- 

 cited tube, and the threads are more strongly negative than before, and their 

 divergence is consequently augmented." 



Pursuing the principle thus developed still further, Franklin now having re- 

 stored the conductor to its natural state, presented the excited glass tube to the 

 tassel. The threads immediately diverged. 



Maintaining the tube in that position with one hand, he presented the finger 

 of the other to the tassel. The threads receded from the finger as if repelled 

 by it. 



This was explained on the same principle. When the excited tube is pre- 

 sented to the tassel, part of the natural electricity of the threads is driven out 

 of them into the conductor, and they are negatively electrified, and therefore 

 repel each other. When the finger is presented to the tassel (being then close 

 to the glass tube), part of its natural electricity is driven back through the 

 hand and body, and the finger becomes, as well as the threads, negatively elec- 

 trified, and so repels, and is repelled by them. To confirm this, hold a slender 

 light lock of cotton, two or three inches long, near a conductor positively elec- 

 trified. You will see the cotton stretch itself out toward the conductor. At- 

 tempt to touch it with the finger of the other hand, and it will be repelled by 

 the finger. Approach it with a positively-charged wire of a bottle, and it will 

 fly to the wire. Bring it near a negatively-charged wire of a bottle, it will 

 recede from that wire in the same manner that it did from the finger, which 

 demonstrates that the finger was negatively electrified as well as the cotton.* 



The great principle thus thrown before the scientific world by Franklin, was 

 immediately taken up and pursued through its consequences by Wilke and 

 ^Epinus, who carried on their researches together at Berlin. The most im- 

 portant result of their combined labors was the invention of the instrument, 

 which, as subsequently improved under the hands of Volta, became the CON- 

 DENSER now so useful in electroscopical investigations. 



In applying the principle of induction to the phenomena of the Leyden jar, 

 and to the same effects as exhibited by the oppositely electrified surfaces of a 

 coated plate of glass, these philosophers saw that the negative state of one sur- 

 face of the glass was, according to the Franklinian theory, the necessary con- 

 sequence of the positive state of the other. This contrary state of the elec- 

 tricities could only be maintained on the supposition that glass was imperme- 

 able by the electric fluid ; and Wilke and jEpinus reasoned, that to whatever 

 extent air or any other body might be similarly impermeable, to the same ex- 

 tent might it be charged on its opposite surfaces. To realize this conception 

 with a plate of air, they coated two large boards of equal size with tin-foil, and 

 suspended them one over the other, leaving a space of about an inch in thick- 

 ness between them. This space was, in fact, a plate of air, of which the up- 

 per and lower surfaces were in contact with the metallic coating of the boards. 

 The lower board communicated with the ground, and a charge of positive 

 electricity was given to the upper one. The lower one then became charged 

 with negative electricity ; and when a person touched at the same time the 



