CHRISTIAN HUYGHENS 59 



of the surrounding ether. In the case of light-giving solids such as 

 red-hot metal or carbon we may suppose this movement to be caused 

 by the rapid motions of the metal or wood, the particles on the surface 

 exciting the ether. Hence the vibration producing light must be 

 much shorter and faster than that causing sound, since we do not find 

 that sound disturbances give rise to light any more than the wave of 

 the hand through the air causes sound. 



The next question is in regard to the nature of the medium through 

 which the vibration produced by light-giving bodies moves. I have 

 named it ether, but it plainly differs from the medium through which 

 sound moves. The latter is simply the air we feel and breathe, and 

 when it is removed from any space, the medium which carries light 

 still remains. This is shown by surrounding the sounding body in a 

 glass vessel, and exhausting the air by means of the air-pump that Mr. 

 Boyle has devised, and with which he has performed so many striking 

 experiments. In trying this experiment, however, it is best to set the 

 sounder on cotton or feathers so that it cannot communicate vibrations 

 to the glass receiver or the air-pump, a point hitherto neglected. 

 Then, when all the air has been exhausted, one catches no sound from 

 the metal when it is struck. 



Hence we conclude not only that our atmosphere which cannot pen- 

 etrate glass is the medium through which sound acts, but that the me- 

 dium carrying light-vibrations is something different: for after the 

 vessel is exhausted of air, light passes through it as easily as before. 



The last point is proven even more conclusively by the famous ex- 

 periment of Torricelli. [Fill a long closed glass tube with mercury, 

 then invert it.] The top of the glass tube not filled by the mercury 

 contains a high vacuum, but transmits light as well as when filled 

 with air. This demonstrates that there is within the tube some form 

 of matter different from air, and which penetrates either glass or 

 mercury, or both, though both are impenetrable to air. And if a like 

 experiment is tried with a little water on top of the mercury, it be- 

 comes equally clear that the substance in question traverses either 

 glass or water or both. 



In regard to the different methods of transmission of sound and 

 light, in the case of sound it is easy to see what happens when one 

 remembers that air can be compressed and reduced to a much smaller 

 volume than usual, and that it tends with the same force to expand to 



