An easy experiment demonstrates that sound cannot travel in a vacuum; that is, sound requires a medium through which to travel. Although a similar experiment shows that light does not require air in which to travel, the Dutch philosopher Huygens first propounded the theory of the existance of an all-pervading medium through which heat and light travel, finding it easier to believe that such an "ether" existed rather than that interplanetary space is almost entirely empty. Early textbooks on the science of wireless continued to describe the "ether." We now know that light and heat can travel though a vacuum and do not require any medium such as an ether.
The bell in the box below is sealed in an airtight chamber. Click the green box on the left to start the bell and also the vacuum pump which gradually removes the air from the chamber. As the pump removes more and more air from the chamber, the amplitude of sound recorded by the microphone diminishes until finally no sound at all is picked up by the microphone. The bell keeps ringing, but no sound is heard by the microphone with no air in the chamber since sound (unlike light) cannot travel in a vacuum.

Author A.T. Story writes about ether waves in his 1904 book called "The Story of Wireless Telegraphy." He states that "...by means of disruptive electrical discharges electromagnetic oscillations are set up in the ether with which all space is filled..." Given the state of knowledge at the time, that sounds quite reasonable. But then Story goes on to note, "They [ether waves] are of course invisible to the unaided eye, though the eye of the camera has seen and transfixed them (Fig. 2). "

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