Faraday, with his curved lines of electric force, and his dielectric efficiency of air and of liquid and solid insulators, resuscitated the idea of a medium through which, and not only through which but _by_ which, forces of attraction or repulsion, seemingly acting at a distance, are transmitted. The long struggle of the first half of the eighteenth century was not merely on the question of a medium to serve for gravific mechanism, but on the correctness of the Newtonian law of gravitation as a matter of fact, however explained. The corresponding controversy in the nineteenth century was very short, and it soon became obvious that Faraday’s idea of the transmission of electric force by a medium not only did not violate Coulomb’s law of relation between force and distance, but that, if real, it must give a thorough explanation of that law. Nevertheless, after Faraday’s discovery of the different specific inductive capacities of different insulators, twenty years passed before it was generally accepted in Continental Europe. But before his death, in 1867, he had succeeded in inspiring the rising generation of the scientific world with something approaching to faith that electric force is transmitted by a medium called ether, of which, as had been believed by the whole scientific world for forty years, light and radiant heat are transverse vibrations. Faraday himself did not rest with this theory of electricity alone. The very last time I saw him at work at the Royal Institution was in an underground cellar, which he had chosen for freedom from disturbance, and he was arranging experiments to test the time of propagation of magnetic force from an electromagnet through a distance of many yards of air to a fine steel needle, polished to reflect light; but no result came from those experiments. About the same time, or soon after, certainly not long before the end of his working time, he was engaged (I believe at the Shot Tower, near Waterloo Bridge, on the Surrey side) in efforts to discover relations between gravity and magnetism, which also led to no result. [Sidenote: KELVIN’S APPRECIATION.] Lord Kelvin, who was himself the first to perceive that Faraday’s ideas were not inconsistent with mathematical expression, and to direct Clerk Maxwell and others to this view, had, in 1854, delighted the old man by bringing mathematical support to the conception of lines of force. In 1857 he sent to Faraday a copy of one of his papers, and received in acknowledgment a letter of warm encouragement, which, however, does not appear to have been preserved. Lord Kelvin’s reply is its own best commentary:--