dred miles away, he might feel the enormous importance of the work that he had accomplished to the world at large, but I much doubt whether he would have felt a more lively satisfaction than when he first saw the electric spark jump between the ends of his coil surrounding the magnet. The chief question which interested Faraday during the greater part of his life was the question of action at a distance. How can the motion of a magnet or what amounts to the same thing, the change of current in one coil, cause a current to flow in another coil in a different place. This he explained by some change in the medium surrounding the coils, but it was reserved for another to give the complete explanation. This was Clerk-Maxwell, of whom I spoke at the beginning, who was the chief expounder of Faraday's views, to which he added and which he made precise by his wonderful ability to put them into mathematical form. It was Maxwell's brilliant idea that the medium which is affected by the presence of an electric current is nothing else than the ether which is supposed to convey the waves of light, and it was a result of his theory that the electric and magnetic actions are transmitted through the ether in the form of waves. Not only this, but he showed that the velocity of these electromagnetic waves would be exactly that of light. He then made the startling generalization that light waves possess all the characteristics of electromagnetic waves, and in fact differ from them in no essential way. These ideas of Maxwell, first put forward nearly fifty years ago, have now found universal acceptance, and the whole world believes that light is an electromagnetic phenomenon. But it was a long time before Maxwell's ideas were accepted, especially on the continent of Europe. For Maxwell died in 1879 without ever having demonstrated experimentally that electric and magnetic effects are propagated in waves. This was reserved for another, the German Heinrich Hertz, who in 1887-88 was able to demonstrate the propagation of such effects with a definite velocity, which was found to be indeed the same as that of light.
Hertz's first experiment by which this discovery was made was so simple that it may be described. If we have two metal spheres near enough together a spark will pass between them if they are electrified, but only if the electrical potential or pressure is different for the two balls. If the two balls form the ends of a circuit of wire, the whole may be electrified as strongly as we please with never a sign of a spark passing between the balls, for the whole conductor has the same potential. But Hertz found that if the wire, in the form of a rectangular circuit, was connected with one of the ends of an induction coil producing sparks, each time that a spark passed from the induction coil a spark also passed between the balls of the rectangle. This was always supposing that the connection was made to a point of the rectangle not symmetrically placed with respect to the balls, and Hertz explained the
