may be done in the case of glass. For instance, if we take a brass
rod in the hand and apply the rubber vigorously, it will fail to
attract the pithball, for there is no trace of electricity upon
it. This is because the metal differs from the glass in another
electrical property, and they must therefore be differently
treated. Brass, in fact, is a conductor of electricity and glass
is not. In other words, electricity is conducted or led away by
brass, so that, as soon as it is generated by the friction, it
flows through the hand and body of the experimenter, which are
also conductors, and is lost in the ground. Glass on the other
hand, is an INSULATOR, and the electricity remains on the surface
of it. If, however, we attach a glass handle to the rod and hold
it by that whilst rubbing it, the electricity cannot then escape
to the earth, and the brass rod will attract the pith-ball.

All bodies are conductors of electricity in some degree, but they
vary so enormously in this respect that it has been found
convenient to divide them into two extreme classes--conductors and
insulators. These run into each other through an intermediate
group, which are neither good conductors nor good insulators. The
following are the chief examples of these classes:--

CONDUCTORS.--All the metals, carbon.

INTERMEDIATE (bad conductors and bad insulators).--Water, aqueous
solutions, moist bodies; wood, cotton, hemp, and paper in any but
a dry atmosphere; liquid acids, rarefied gases.

INSULATORS.--Paraffin (solid or liquid), ozokerit, turpentine,
silk, resin, sealing-wax or shellac, india-rubber, gutta-percha,