24 



CHEMISTRY. 



very large jar, by which great extent of surface and an enormous 

 accumulation of electricity are gained ; the whole may be discharged 

 at the same moment, — and the effect is, of course, exceedingly 

 powerful. 



By electrical induction is meant the power which an electrified 

 body has to produce an oppositely electric condition in a contiguous 

 body ; it is by virtue of this law, that when an electrified body, as a 

 glass tube previously rubbed, approaches a light body, as a feather 

 or pith-ball, it immediately attracts it, having first induced in it an 

 opposite electrical state. A series of globes suspended by silk 



threads, in the manner repre- 



Fig. 9. 



sented, will each become 

 electric by induction^ when 

 a charged body is brought 

 near the end of the series. 

 The positive and negative 

 signs are intended to repre- 

 sent the states. 



The electrical state of the 

 atmosphere is liable to disturbance : experiment has shown that the 

 higher regions of the air are usually in a positive state : in cloudy 

 and stormy weather, the clouds near the surface often appear in a 

 negative state. In a thunder-storm, the cloud and the earth may be 

 considered as representing the two coatings of the Leyden jar, dif- 

 ferently electrified, — and the intervening air, the bad conducting 

 glass. The dangerous effects of lightning are much lessened by the 

 use of lightning rods^ which are metallic conductors terminating 

 above in a point, and' below passing to a considerable depth 

 into the earth. The object of the 'pointed extremity is to conduct 

 off the discharge silently ; a blunt extremity would give 

 rise to a spark, and perhaps a shock, which might 

 be unsafe ; this may be easily shown by experiment 

 with the electrical machine. 



Electrometers and electroscopes are instruments for 

 indicating or measuring the electrical intensity. The 

 most simple one is that named the gold leaf electrome- 

 ter; it consists of a pair of gold leaves suspended from 

 the top of a bell -jar and communicating above with a 

 metallic cap. When an electrified body is brought 

 near to the cap, its presence is immediately detected 

 by the divergence of the gold leaves. There are 

 other electrometers known by the names of quadrant 

 electrometer^ torsion electrometer^ balance electrometer, &c. 



SECTION IV. 

 GALVANISM, OR VOLTAIC ELECTRICITY. 

 When two solid bodies, as two pieces of metal, are plunged into a 



