ON THE OKiaiN OF THE PLANETARY SYSTEM. 165 



dust, are still everywhere scattered; at any rate, in 

 those parts of space which our earth traverses. 



The existence of the luminiferous ether cannot be 

 considered doubtful. That light and radiant heat are 

 due to a motion which spreads in. all directions has been 

 sufficiently proved. For the transference of such a 

 motion through space there must be something which 

 can be moved. Indeed, from the magnitude of the 

 action of this motion, or from that which the science 

 of mechanics calls its vis viva, we may indeed assign 

 certain limits for the density of this medium. Such 

 a calculation has been made by Sir W. Thomson, the 

 celebrated Glasgow physicist. He has found that the 

 density may possibly be far less than that of the air 

 in the most perfect exhaustion obtainable by a good 

 air-pump ; but that the mass of the ether cannot be 

 absolutely equal to zero. A volume equal to that of 

 the earth cannot contain less than 2,775 pounds of 

 luminous ether. 1 



The phenomena in celestial space are in conformity 

 with this. Just as a heavy stone flung through the 

 air shows scarcely any influence of the resistance of 

 the air, while a light feather is appreciably hindered ; 

 in like manner the medium which fills space is far too 

 attenuated for any diminution to have been perceived 



1 This calculation would, however, lose its bases if Maxwell's hypo- 

 thesis were confirmed, according to which light depends on electrical 

 and magnetical oscillations 



