30 PHYSICS. 
ELECTRICITY. 
ELEcTRIcITY received its name from the Greek word élektron, amber, 
in which substance it was first detected. It was found, from the earliest 
times, that when a piece of amber was rubbed with a dry cloth, it had 
the power of attracting small light bodies lying near it. In later times, ~ 
it was observed by scientific men that sulphur, glass, sealing-wax, and 
many other substances had the same property ; and now-a-days electric 
experiments are all made with these common substances, instead of the 
comparatively rare and costly amber, especially since they exhibit the 
phenomena of electricity in as great perfection as the substance in which 
it was first discovered. 
In order to observe better what takes place, let us suspend something 
very light—a little ball of pith of the elder-tree is generally used— 
by a silk thread from a glass tube, as in the 
figure (the reason why glass and silk are 
an used will be explained afterwards). If a glass 
‘ tube be rubbed with a piece of dry silk, and 
& then held near the pith-ball, the latter will 
, 4 at first be drawn to the glass, and then be 
‘ 1 driven away, as indicated in the figure. If a 
\\ stick of sealing-wax be taken next, rubbed 
\\ with a piece of flannel, and held to the ball, 
which has just been repelled by the glass, the 
== same thing will take place—the ball will 
first be attracted, and then repelled. If the 
Fig. 32. class be rubbed again, the same thing may be 
repeated of course; so with the wax; and 
the pith-ball might thus be kept playing between the two for any length 
of time. There are thus two kinds of electricity, one produced in glass 
when rubbed with a piece of silk; and the other in sealing-wax when 
rubbed with a woollen cloth. When the electrified glass attracts the 
pith- -ball, electricity is communicated to the ball, and thon it is repelled ; 
in other words, two bodies charged with the same kind of electricity 
repel each other. Then the ball is attracted by the wax, and is only 
repelled again when the electricity received rom the glass has been 
“replaced ss the kind produced in the wax ; and we infer from this that two 
bodies charged with the different kinds e electricity attract each other. 
Special names have been given to these two kinds of electricity : the kind 
produced in glass and a number of other substances is called vitreous, 
from Latin vitrum, glass; while that produced in sealing-wax and a 
number of other substances of a resinous nature is called resinous, 


