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.