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.