It will be observed that I have spoken in the preceding paragraph of the passage of a current along the wire connecting the two poles of a powerful electric battery, or along any substance connecting those poles which possesses the property of being what is called a good conductor of electricity. But the reader is not to assume that there is such a current, or that it is known to flow either from the positive to the negative pole, or from negative to positive pole; or, again, that, as some have suggested, there are two currents which flow simultaneously in opposite directions. We speak conventionally of the current, and for convenience we speak as though some fluid really made its way (when the circuit is complete) from the positive to the negative pole of the compound battery. But the existence of such a current, or of any current at all, is purely hypothetical. I should be disposed, for my own part, to believe that the motion is of the nature of wave-motion, with no actual transference of matter, at least when the circuit is complete. According to this view, where resistance takes place we might conceive that the waves are converted into rollers or breakers, according to the nature of the resistance--actual transference of matter taking place through the action of these changed waves, just as waves which have traversed the free surface of ocean without carrying onward whatever matter may be floating on the surface, cast such matter ashore when, by the resistance of the shoaling bottom or of rocks, they become converted either into rollers or into breakers. I may also notice, with regard to good conductors and bad conductors of electricity, that they may be compared to substances respectively transparent and opaque for light-waves, or again, to substances which allow heat to pass freely or the reverse. Just as light-waves fail to illuminate a transparent body, and heat-waves fail to warm a body which allows them free passage, so electricity-waves (if electricity really is undulatory, as I imagine) fail to affect any substance along which they travel freely. But as light-waves illuminate an opaque substance, and heat-waves raise the temperature of a substance which impedes their progress, so waves of electricity, when their course is impeded, produce effects which are indicated to us by the resulting heat and light.