192 GENERAL SCIENCE 



rubbed with a piece of silk also acquires this power of 

 attraction. It is thought that a Greek philosopher 

 named Thales discovered this property in a substance 

 called amber, and for many years amber was supposed 

 to be the only substance possessing this property. Since 

 amber in common with a few metals was called " elec- 

 tron " by the Greeks, Dr. Gilbert of England many 

 years later gave the name " electric " to these phe- 

 nomena. 



Place some bits of paper under a pane of glass which 

 is supported by two books (Figure 160). Rub the upper 



FIG. 160. 



surface of the glass with a piece of silk and notice how 

 the bits of paper are disturbed. 



Two Kinds of Electricity. - - The electricity which is 

 produced upon glass by rubbing it with silk is called posi- 

 tive electricity and that produced upon the rubber is 

 called negative electricity. Electricity is usually pro- 

 duced when two unlike substances are rubbed together, 

 and the kind may be determined by comparing it with 

 that formed on glass or rubber. Thus all electrified 

 bodies which act with respect to other bodies like a glass 

 rod rubbed with silk are said to be positively electrified, 

 while those that act like a piece of hard rubber that has 

 been rubbed with the woolen cloth are said to be nega- 

 tively electrified. 



