These theories and the late discovery of Hertz that electrical energy manifests itself in the form of waves, oscillations or vibrations, similar to light, but not so rapid as the vibrations of light, constitute about all that is known about the nature of this force. Franklin believed it was a single fluid, but others taught that there were two kinds of electricity, positive and negative, that the like kinds were repulsive and the unlike kinds attractive, and that when generated it flowed in currents. Such terms are not now regarded as representing actual varieties of this force, but are retained as convenient modes of expression, for want of better ones, as expressing the conditions or states of electricity when produced. Electricity produced by friction, that is, developed upon the surface of a body by rubbing it with a dissimilar body, and called frictional or static electricity, was the only kind produced artificially in the days of Franklin. What is known as galvanism, or animal electricity, also takes its date in the 18th century, to which further reference will be made. Since 1799 there have been discovered additional sources, among which are voltaic electricity, or electricity produced by chemical action, such as is manifested when two dissimilar metals are brought near each other or together, and electrical manifestations produced by a decomposing action, one upon the other through a suitable medium; inductive electricity, or electricity developed or induced in one body by its proximity to another body through which a current is flowing; magnetic electricity, the conversion of the power of a magnet into electric force, and the reverse of this, the production of magnetic force by a current of electricity; and thermal electricity, or that generated by heat. Electricity developed by these, or other means in contra-distinction to that produced by friction, has been called dynamic; but all electric force is now regarded as dynamic, in the sense that forces are always in motion and never at rest. Many of the manifestations and experiments in later day fields which, by reason of their production by different means, have been given the names of discovery and invention, had become known to Franklin and others, by means of the old methods in frictional electricity. They are all, however, but different routes leading to the same goal. In the midst of the brilliant discoveries of modern times confronting us on every side we should not forget the honourable efforts of the fathers of the science. We need not dwell on what the ancients produced in this line. It was a single fact only:--The Greeks discovered that amber, a resinous substance, when rubbed would attract lighter bodies to it.