SCIENTIFIC USE OF THE IMAGINATION. 135 



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 hesita 

 tion the Undulatory Theory of Light. Like you and me, 

 they one and all believe in an ether and its light-producing 

 waves. Let us consider what this belief involves. Bring 

 your imaginations once more into play and figure a series 

 of sound-waves passing through air. Follow them up to 

 their origin, and what do you there find ? A definite, tan 

 gible, vibrating body. It may be the vocal chords of a 

 human being, it may be an organ-pipe, or it may be a 

 stretched string. Follow in the same manner a train of 

 ether-waves to their source ; remembering at the same time 

 that your ether is matter, dense, elastic, and capable of 

 motions subject to and determined by mechanical laws. 

 What then do you expect to find as the source of a series 

 of ether-waves ? Ask your imagination if it will accept a 

 vibrating multiple proportion a numerical ratio in a state 

 of oscillation ? I do not think it will. You cannot crown 

 the edifice by this abstraction. The scientific imagination, 

 which is here authoritative, demands as the origin and cause 

 of a series of ether-waves a particle of vibrating matter 

 quite as definite, though it may be excessively minute, as 

 that which gives origin to a musical sound. Such a particle 

 we name an atom or a molecule. I think the seeking intel 

 lect when focussed so as to give definition without penum- 

 bral haze, is sure to realize this image at the last. 



With a view of preserving thought continuous through 

 out this discourse, and of preventing either failure of knowl- 



