164 ON THE ORIG-IN OF THE PLANETAEY SYSTEM. 



resistance is mainly due to that of the particles of air 

 against each other, due to their friction against the 

 wheel. 



If we could once set a body in rotation, and keep 

 it from falling, without its being supported by another 

 body, and if we could transfer the whole arrangement to 

 an absolute vacuum, it would continue to move for ever 

 with undiminished velocity. This case, which cannot 

 be realised on terrestrial bodies, is apparently met 

 with in the planets with their satellites. They appear 

 to move in the perfectly vacuous cosmical space, with- 

 out contact with any body which could produce 

 friction, and hence their motion seems to be one which 

 never diminishes. 



You see, however, that the justification of this 

 conclusion depends on the question whether cosmical 

 space is really quite vacuous. Is there nowhere any 

 friction in the motion of the planets ? 



From the progress which the knowledge of nature 

 has made since the time of Laplace, we must now 

 answer both questions in the negative. 



Celestial space is not absolutely vacuous. In the 

 first place, it is filled by that continuous medium the 

 agitation of which constitutes light and radiant heat, 

 and which physicists know as the luminiferous ether. 

 In the second place, large and small fragments of 

 heavy matter, from the size of huge stones to that of 



