ebonite, ivory, dry wood, dry glass or porcelain, mica, ice, air
at ordinary pressures.

It is remarkable that the best conductors of electricity, that is
to say, the substances which offer least resistance to its
passage, for instance the metals, are also the best conductors of
heat, and that insulators made red hot become conductors. Air is
an excellent insulator, and hence we are able to perform our
experiments on frictional electricity in it. We can also run bare
telegraph wires through it, by taking care to insulate them with
glass or porcelain from the wooden poles which support them above
the ground. Water, on the other hand, is a partial conductor, and
a great enemy to the storage or conveyance of electricity, from
its habit of soaking into porous metals, or depositing in a film
of dew on the cold surfaces of insulators such as glass,
porcelain, or ebonite. The remedy is to exclude it, or keep the
insulators warm and dry, or coat them with shellac varnish, wax,
or paraffin. Submarine telegraph wires running under the sea are
usually insulated from the surrounding water by india-rubber or
gutta-percha.

The distinction between conductors and non-conductors or
insulators was first observed by Stephen Gray, a pensioner of the
Charter-house. Gray actually transmitted a charge of electricity
along a pack-thread insulated with silk, to a distance of several
hundred yards, and thus took an important step in the direction of
the electric telegraph.

It has since been found that FRICTIONAL ELECTRICITY APPEARS ONLY
ON THE EXTERNAL SURFACE OF CONDUCTORS.