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physical action can take place without the intervention of physical causes is repugnant to the human mind, and therefore physicists have invented the ether. By this they mean a physical something which pervades all space, whether filled by bodies or not, and this ether forms the connecting link by which forces are transmitted across space. Once we assume the existence of this physical though imponderable, that is, weightless substance, we may regard it as instrumental not only in the transmission of gravitational forces, but also of electric and magnetic forces and as the carrier of light and heat rays. There is, indeed, very strong experimental evidence that light, electricity, magnetism and all other manifestations of energy are propagated by ether vibrations. Maxwell was the first to point out that a connection of this kind exists, and by adopting his "electromagnetic theory of light" it can be proved mathematically, what has also been experimentally verified, that the speed at which a telegraph signal travels along the wire is equal to the speed of light propagation. It is extremely unlikely that such an agreement should be a mere coincidence, and we are therefore justified in assuming that the