USE OP THE IMAGINATION. 423 



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 " mul- 

 tiple 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 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, tangible, 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 ex- 

 pect 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 

 with 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 a"n atom or a molecule. I 

 think the intellect, when focused so as to give definition 

 without penumbral haze, is sure to realize this image at 

 the last. 



With the view of preserving thought continuous 

 throughout this discourse, and of preventing either failure 

 of knowledge or of memory, from causing any rent in our 

 picture, I here propose to run rapidly over a bit of ground 

 which is probably familiar to most of you, but which I am 

 anxious to make familiar to you all. The waves generated 

 in the ether by the swinging atoms of luminous bodies are 

 of different lengths and amplitudes. The amplitude is the 



