96 
Annual Report of the 
that is, may become charged with either the positive or negative 
fluid in excess of its natural share, If, for example, smooth glass 
be rubbed with woolen or silk cloth, the two fluids are separated; 
the positive fluid being accumulated on the glass, and the negative 
fluid in equal quantity on the cloth, as careful experiment proves. 
This excitement is shown by each body’s attracting light substances, 
such as a pith-ball suspended by a silk thread. Now, if this pith- 
ball be touched by the excited glass, it will become electrified, that 
is, will receive free electricity from the excited body, and will be 
instantly repelled. The same takes place when sealing wax is 
rubbed with a woolen or silk cloth. That there are two kinds of 
electricity is obvious from the fact that two pith-balls, when both are 
electrified by contact with the excited glass or sealing wax, mu¬ 
tually repel each other; but if one be electrified by contact with 
the glass and the other with the wax, they mutually attract. So 
all bodies are susceptible of being excited by friction with either 
positive or negative electricity. 
To illustrate the difference between a conductor and non-conduc¬ 
tor, let us suppose a globe mounted upon a glass standard. If the 
surface of the glass be free from moisture, and the air dry, this 
globe is said to be insulated , that is, surrounded by non-conductors. 
Suppose now, the globe to be touched by an excited body; if the 
globe be a good conductor, as metal, free electricity will instantly 
be diffused over its whole surface; if the globe be a non-conductor, 
as glass, it will become electrified only at, or a little beyond the 
point in actual contact with the excited body. So, if the electri¬ 
fied globe be touched with the finger, or connected with the 
ground by a good conductor, it will, if of metal, instantly part with 
all its free electricity; but if of glass, only that at or near the point 
in contact with the finger. Thus we may determine the character 
of different substances as conductors of electricity, by the time an 
electrified body is losing its free electricity when touched by rods 
formed of these different substances. 
A body, insulated as above explained, will retain its free electric¬ 
ity for some time. If there be moisture on the surface of the glass 
standard, or in the air, it will soon conduct off the free electricity. 
If it be perfectly insulated, it will gradually lose its charge by con¬ 
nection, that is, the particles of air in contact with the body 
become electrified and then are repelled. These are followed by 
