THE USE OF HYPOTHESIS. 155 



eally entertained it did not do more than describe what 

 ook place. It involved no deep analogy to any other phe- 

 lomena of nature, for Newton could not point to any 

 ther substance which went through these extraordinary 

 hanges. We now know that the true analogy would 

 lave been the waves of sound, of which Newton had 

 icquired in other respects so complete a comprehension. 

 But though the notion of interference of waves had dis- 

 inctly occurred to Hooke, Newton had failed to see how 

 he periodic phenomena of light could be connected with 

 he periodic character of waves. His hypothesis fell be- 

 siuse it was out of analogy with everything else in nature, 

 md it therefore did not allow him, as in other cases, to 

 lescend by mathematical deduction to consequences which 

 'ould be verified or refuted. 



We are always at freedom again to imagine the existence 

 f a new agent or force, and give it an appropriate name, 

 >rovided there are phenomena incapable of explanation 

 rom known causes. We may speak of vital force as oc- 

 lasioning life, provided that we do not take it to be more 

 han a name for an undefined something giving rise to 

 nexplicable facts, just as the French chemists called Iodine 

 he Substance X, while they were unaware of its real 

 haracter and place in chemistry >". Encke was quite 

 ustified in speaking of the resisting medium, in space so 

 ong as the retardation of his comet could not be other- 

 wise accounted for. But such hypotheses will do much 

 harm whenever they divert us from attempts to reconcile 

 ithe facts with known laws, or when they lead us to mix 

 up entirely discrete things. We have no right, for 

 \ instance, to confuse Encke' s supposed resisting medium, 

 with the ethereal basis of light. The name protoplasm, 

 now so familiarly used by physiologists, is doubtless 

 legitimate so long as we do not mix up different sub- 



y Paris, 'Life of Davy,' p. 274. 



