rubbing glass with leather. At first sight, it does not seem as if the
two counter-substances represent the required alchemic counter-poles to
resin and glass. For both hair and leather are animal products and
therefore seem to be of like nature. Closer inspection, however, shows
that they do obey the rule. For hair, like all horny substances, is a
dead product of external secretion by the animal organism. An
ur-phenomenal example of it, showing its kinship to glass-like
substances, is the transparent cornea of the eye, close to the
crystal-lens. Leather, on the other hand, is a product of the
hypodermic part of the body and, as such, belongs to those parts of the
organism which are filled with blood, and, therefore, permeated with
life. (Note as a characteristic of leather that it requires a special
treatment, tanning, to make it as immune from decay as hair is by
nature.) Hair and leather, therefore, represent in themselves a
salt-sulphur polarity, and thus fulfil the corresponding function when
brought together with resin or glass respectively.

What is true for the particular substances which originally led man to
discover the dual nature of electricity, holds good equally for any
pair of substances capable of assuming the electric state when rubbed
against each other. If we examine from this point of view the series of
such substances, as usually given in the textbooks on electricity, we
shall always find a substance of extreme salt-character at the one end,
and one of extreme sulphur-character at the other, the substances as a
whole forming a gradual transition from one extreme to the other. Which
kind of electricity appears on each, when submitted to friction,
depends on whether the counter-substance stands on its right or left,
in the series. It is the particular relation between the two which
makes them behave in one way or the other.