ELECTllICITY. 149 



when handled in the dark, frequently exhibit a sparkling 

 appearance, attended with the same kind of noise that is 

 heard in the experiment of the glass tube. 



All those bodies which transmit or conduct electricity 

 from one surface to another, are called conductors, and those 

 surfaces that will not transmit the electric power, are called 

 electrics or non-conductors. The general class of conduc- 

 tors comprehends metals, ores, and fluids in their natural 

 state, except air and oils. Vitrified and resinous substances, 

 amber, sulphur, wax, silk, cotton, and feathers, are electrics 

 or non-conductors. Many of these, such as glass, resin,, 

 and air, become conductors by being heated. When a sur- 

 face is supposed to have more than its natural quantity of 

 this fluid, it is said to be positively electrified ; and when 

 less than its natural share, to be negatively electrified. When 

 any electrified conductor is wholly surrounded by non-con- 

 ductors, so that the electric fluid cannot pass from it along 

 conductors to the earth, it is said to be insulated. The hu- 

 man body is a good conductor of electricity ; but if a person 

 stand on a cake of resin, or on a stool supported by glass 

 legs, the electric fluid cannot pass from him to the earth, 

 and if he is touched by another person standing on the 

 ground, the same sparkling appearance and noise, as men- 

 tioned above, will be exhibited. Two surfaces, both posi- 

 tively, or both negatively electrified, repel each other ; and 

 two substances, of which one is positively, and the other 

 negatively electrified, attract each other. Opposite electri- 

 cities always accompany each other, for if any surface be- 

 come positive, the surface with which it is rubbed becomes 

 negative ; and if any surface be rendered positive, the near- 

 est conducting surface will become negative. When one 

 side of a conductor receives the electric fluid, its whole sur- 

 face is instantly pervaded ; but when an electric or non- 

 conductor is presented to an electrified body, it becomes 

 electrified on a small spot only. If to one side of a pane of 

 glass, you communicate positive electricity, the opposite side 

 will become negatively electrified, and the plate is then said 

 to be charged. These electricities cannot come together, 

 unless a communication, by means of conductors, is made 

 between the sides of the glass; and if their union be made 

 through the human body, it produces an aifection of the 

 nerves called an electric shock. 

 13* 



