But where has the energy gone? This question is just as important in this case as it was in the case of the tuning-fork. The old electrical theories answered this question one way, and Maxwell, inspired by Faraday, answered it in another. The old theories maintained that there is no other electrical motion except the motion of the charges along the conducting surface of the spheres and the rods. They paid no attention to the motion of the lines of force, because they knew nothing about them. Their vision did not see the lines themselves but only their terminals, the charges. Hence, according to the old theories, all of the energy imparted by the machine is transformed into heat in the conducting parts of the oscillator. Hertz was the first to prove that a part of the energy is radiated off into space, in a similar manner as the energy of a tuning-fork is radiated off in the form of sound waves. He detected in the space surrounding the oscillator the presence of electrical waves, that is, periodically recurring variations of the density of the electrical lines of force; he measured their length, and, having calculated the period of his oscillator, he divided the wave-length by the period and obtained the velocity of propagation. It came out, in his earliest experiments, roughly equal to the velocity of light, as the Faraday-Maxwell theory had predicted. The waves were reflected and refracted by insulators denser than air, and all these and other effects Hertz demonstrated to follow the laws which hold good for light, supporting admirably Maxwell’s theory that light is an electromagnetic disturbance. Even this preliminary report which Hertz had sent to Helmholtz convinced everybody that the Faraday-Maxwell electromagnetic theory had triumphed, and that our knowledge of electromagnetic phenomena had been wonderfully extended. Subsequent experiments by Hertz and others added more and more laurels to this first victory. That meeting of the Physical Society in Berlin was what I always considered the inauguration day of the electromagnetic theory. Prior to that day the theory existed in all its beautiful completeness, but it dwelt on high in the celestial heights of Faraday and Maxwell. Continental physicists needed the guidance of a Helmholtz to reach these heights. After that day it came down to earth and lived among mortal men and became part of their mode of thought. It was a heavenly gift which Hertz brought down to earth. Everybody was convinced that the science of light had become a part of the science of electricity. _This new knowledge was the second great revelation of the nineteenth century. The wonderful things which followed in its wake, even before the nineteenth century had closed, testify to the greatness of that revelation._