108 FKAGMENTS OF SCIENCE. 



vinced of this as of anything. What is your warrant 

 for this conviction? Simply and solely this: your 

 fellow-creatures behave as if they were reasonable; the 

 hypothesis, 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 ether, 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 ether had the best of it. 



This universal medium, this light-ether 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 neces- 

 sary. 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 hesi- 



