148 PHYSICS. 
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measure the intensity of the electricity by the divergence of the bits of sts aw 
or gold leaf. This divergence is measured along a graduated arc attached 
to the glass cover. In the best of these instruments a drawer, sliding into 
the bottom, contains chloride of lime, for the purpose of keeping the 
inclosed air perfectly dry. 
Electricity is confined to the surface of bodies, penetrating below to an 
entirely inappreciable extent. This is shown by the following experi- 
ment :—Electrify an insulated metallic ball (pl. 22, fig. 60), and fit to it 
two hollow metallic hemispheres with glass handles. Suddenly removing 
these hemispheres, they will be found to contain all the electricity, all 
traces having vanished from the ball. 
If an electric pendulum or pith ball electroscope (pl. 20, fig. 22) be 
brought near an excited glass tube or rod of sealing wax, the pith ball of the 
former will first be attracted to the tube, and after contact immediately 
repelled, this repulsion continuing until the ball is touched by some 
conductor. Hence we conclude that electrified and unelectrified bodies 
attract each other. This attraction and repulsion are well illustrated in the 
electric dance (pl. 22, fig. 72). Here two metallic plates are required 
one suspended from the prime conductor of an electric machine by a brass 
chain, the other supported on a conducting stand at a short distance 
immediately below the first. Little figures, made of elder pith or paper, 
are then to be placed on the lower plate. When the upper plate is 
electrified the figures will be attracted to it, and receiving a portion of free 
electricity, will be immediately repelled to and attracted by the lower 
plate. Here giving off their free electricity, they are again in a condition 
to be attracted by the upper plate, and the dance can thus be maintained 
for any length of time. 
If we take two pith ball electroscopes (pl. 20, fig. 24), and electrify one 
from an excited glass tube, and the other from sealing-wax, instead of 
repelling each other, as would have been the case if both had received 
electricity from either the glass or the sealing-wax, an actual and mutual 
attraction will ensue. For this reason we are entitled to assume a 
difference in the electricity of glass and resin, and consequently the 
existence of two kinds of electricity. These have been named, respectively, 
_wltreous and resinous, or positive (+) and negative (—). For some time 
the theory of Franklin, that there was but one kind of electricity, an 
excess of which was equivalent to the vitreous, and a deficiency to the 
resinous electricity, was preferred by scientific men to that which assumed 
the existence of two distinct fluids. This latter theory, that of Dufay, as 
modified by Symmer, is now more generally adopted than the other. 
According to this theory the fluids are combined in the ordinary condition 
of a body. If, however, the body be rubbed by the proper substances, this 
equilibrium is disturbed, one of the fluids passing into the rubber, the other 
remaining in the original body. Rubber and rubbed will always then be 
in opposite conditions of electricity, the same body with different 
rubbers being capable of presenting successively the phenomena of either 
fluid. 
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