Originally the evoking of the electric condition was ascribed solely to
the nature of amber, the only substance known to possess this property.
To-day we know that not the amber alone, but its coming together with
another substance of different nature, in this instance an animal
substance of the nature of hair or silk, is required. Whatever
substances we use for friction, they must always be different in
nature, so as to allow both kinds of electricity to appear at once.
Which of the two kinds imposes its presence the more strongly upon the
observer depends on purely extraneous conditions which have nothing to
do with the process itself.

Obviously, if we wish to understand the qualitative difference between
the two kinds of electricity, we must investigate the qualitative
difference in the material substances, which give rise to electricity
when they are rubbed together. We shall again follow the historical
line by examining the two substances which first taught man the polar
nature of electricity. They are glass and resin, after which, as we
mentioned, the two electricities were even named in the beginning.

Our functional conception of matter, developed earlier (Chapter XI),
allows us to recognize in these two substances representatives of the
Salt-Sulphur polarity. Indeed, glass as a mineral substance, which
actually owes its specific character to the presence of silicon in it,
clearly stands on the phosphoric-crystalline side, while resin, being
itself a sort of 'gum', on the sulphurous-volcanic side. In fact,
sulphur itself was soon found to be a particularly suitable substance
for producing 'resin'-electricity.

Now the usual way of producing one kind of electricity is by rubbing
resin (or sulphur, or ebonite) with wool or fur, and the other by