charged ball or indicator, they are of opposite kinds. To
distinguish the two sorts, one is called POSITIVE and the other
NEGATIVE electricity.

Further experiments with other substances will show that sometimes
the rod is negative while the rubber is positive. Thus, if we rub
the glass rod with cat's fur instead of silk, we shall find the
glass negative and the fur positive. Again, if we rub a stick of
sealing-wax with the silk handkerchief, we shall find the wax
negative and the silk positive. But in every case one is the
opposite of the other, and moreover, an equal quantity of both
sorts of electricity is developed, one kind on the rod and the
other on the rubber. Hence we conclude that EQUAL AND OPPOSITE
QUANTITIES OF ELECTRICITY ARE SIMULTANEOUSLY DEVELOPED BY
FRICTION.

If any two of the following materials be rubbed together, that
higher in the list becomes positively and the other negatively
electrified:--

POSITIVE (+).

Cats' fur.
Polished glass.
Wool.
Cork, at ordinary temperature.
Coarse brown paper.
Cork, heated.
White silk.
Black silk.