108 FEAGMENTS OF SCIENCE. 



this conviction ? Simply and solely this : your fellow- 

 creatures behave as if they were reasonable ; the hypo- 

 thesis, for it is nothing more, accounts for the facts. 

 To take an eminent example : you believe that our 

 President is a reasonable being. Why ? There is no 

 known method of superposition by which any one of us 

 can apply himself intellectually to any other, so as to 

 demonstrate coincidence as regards the possession of 

 reason. If, therefore, you hold our President to be 

 reasonable, it is because he behaves as if he were 

 reasonable. As in the case of the aether, beyond the 

 'as if ' you cannot go. Nay, I should not wonder if a 

 close comparison of the data on which both inferences 

 rest, caused many respectable persons to conclude that 

 the aether had the best of it. 



This universal medium, this light-aether as it is 

 called, is the vehicle, not the origin, of wave-motion. 

 It receives and transmits, but it does not create. 

 Whence does it derive the motions it conveys ? For 

 the most part from luminous bodies. By the motion of 

 a luminous body I do not mean its sensible motion, such 

 as the flicker of a candle, or the shooting out of red 

 prominences from the limb of the sun. I mean an 

 intestine motion of the atoms or molecules of the 

 luminous body. But here a certain reserve is necessary. 

 Many chemists of the present day refuse to speak of 

 atoms and molecules as real things. Their caution 

 leads them to stop short of the clear, sharp, mechanically 

 intelligible atomic theory enunciated by Dalton, or any 

 form of that theory, and to make the doctrine of 

 ' multiple proportions ' their intellectual bourne. I 

 respect the caution, though I think it is here misplaced. 

 The chemists who recoil from these notions of atoms 

 and molecules accept, without hesitation, the Un- 

 dulatory Theory of Light. Like you and me they one 



