or nearly closed hollow conductor, an equal amount of the same kind of electricity appeared on the outside of the hollow conductor, while an equal amount of the opposite kind appeared on the interior surface of the conductor. With the ice-pail and the butterfly-net he showed that there could be no free electricity on the interior of a conductor. Lines of force cannot pass through the material of a conductor without producing electric displacement. Every element of electricity must be joined to an equal amount of the opposite kind by a line of force. Such lines cannot pass through the conductor itself; hence the charge must be entirely on the outside of the conductor, so that every element of the charge may be associated with an equal amount of the opposite electricity upon the surfaces of surrounding objects. Thus to Faraday every electrical action was an exhibition of electric induction. All this work had been done before by Henry Cavendish, but neither Faraday nor any one else knew about it at the time. From the fact that there could be no electricity in the interior of a hollow conductor, Cavendish deduced, in the best way possible, the truth of the law of inverse squares as applied to electrical attraction and repulsion, and thus laid the foundation of the mathematical theory of electricity. To Cavendish every electrical action was a displacement of an incompressible fluid which filled the whole of space, producing no effect in conductors on account of the freedom of its motion, but producing strains in insulators by displacing the material of the body. Faraday, in his lines of force, saw, as it were, the lines along which the displacements of Cavendish's fluid took place. Faraday thought that, if he could show that electric induction could take place along curved lines, it would prove that the action took place through a medium, and not directly at a distance. He succeeded in experimentally demonstrating the curvature of these lines; but his conclusions were not warranted, for if we conceive of two or more centres of force acting directly at a distance according to the law of inverse squares, the resultant lines of force will generally be curved. Of course, this does not prove the possibility of direct action at a distance, but only shows that the curvature of the lines is as much a consequence of the one hypothesis as of the other.