ETHER AND PONDERABLE MATTER 



something that undulates; and this something could 

 not be air, for air exists only in infinitesimal quantity, if 

 at all, in the interstellar spaces, through which light 

 freely penetrates. But if not air, what then? Why, 

 clearly, something more intangible than air ; something 

 supersensible, evading all direct efforts to detect it, yet 

 existing everywhere in seemingly vacant space, and also 

 interpenetrating the substance of all transparent liq- 

 uids and solids, if not, indeed, of all tangible sub- 

 stances. This intangible something Young rechrist- 

 ened the Luminiferous Ether. 



In the early days of his discovery Young thought of 

 the undulations which produce light and radiant heat as 

 being longitudinal a forward and backward pulsation, 

 corresponding to the pulsations of sound and as such 

 pulsations can be transmitted by a fluid medium with 

 the properties of ordinary fluids, he was justified in 

 thinking of the ether as being like a fluid in its proper- 

 ties, except for its extreme intangibility. But about 

 1818 the experiments of Fresnel and Arago with polar- 

 ization of light made it seem very doubtful whether the 

 theory of longitudinal vibrations is sufficient, and it 

 was suggested by Young, and independently conceived 

 and demonstrated by Fresnel, that the luminiferous 

 undulations are not longitudinal, but transverse; and 

 all the more recent experiments have tended to con- 

 firm this view. But it happens that ordinary fluids- 

 gases and liquids cannot transmit lateral vibrations; 

 only rigid bodies are capable of such a vibration. So it 

 became necessary to assume that the luminifcTous ether 

 is a body possessing elastic rigidity a familiar property 

 of tangible solids, but one quite unknown among fluids. 



