40 ELEMENTS OF STATIC ELECTRICITY, 



ceivable velocity, suddenly forces the resisting air into 

 the phases of the electric waves ; then the generation 

 of those other modes of motion, known as heat and 

 light, might easily be the result. 



A sharp sound, varying from an insignificant snap to 

 a deafening report, always accompanies the spark. On 

 the condensation theory this is accounted for by the 

 sudden displacement and reflux of the air. But since 

 sound, like heat and light, is another mode of motion, 

 occurring chiefly in the air, it is evident that the wave 

 theory will best account for it ; the electric impulse 

 giving rise to these different modes of motion. 



THE DUAL THEORY. We have already seen, in ex- 

 periments with the pith ball electroscope, that the balls 

 may be attracted and repelled by electrified glass, seal- 

 ing- wax, and various other substances. 



Let an electrified glass rod approach one of the balls ; 

 the ball is attracted, and, after contact, repelled. Now 

 let an electrified stick of sealing-wax be brought near, 

 and the electrified ball, which was repelled by the glass, 

 is attracted by the wax. Or let the ball be first elec- 

 trified and repelled by the wax, and it is attracted by 

 the glass. 



Further experiments show that the same results can 

 be produced with other substances; glass representing 

 a certain class of substances, which show similar electri- 

 fication, and sealing-wax and resin another class, which 

 shows opposite electrification to that of glass. 



Hence it has been assumed that there are two kinds of 

 electricity. One kind generated on the class of sub- 

 stances represented by glass, and the other on the class 

 represented by resin. The former was once designated 

 as vitreous, and the latter as resinous; but the term 



