THE USE OF HYPOTHESIS. 155 



really entertained it did not do more than describe what 

 took place. It involved no deep analogy to any other phe 

 nomena of nature, for Newton could not point to any 

 other substance which went through these extraordinary 

 changes. We now know that the true analogy would 

 have been the waves of sound, of which Newton had 

 acquired in other respects so complete a comprehension. 

 But though the notion of interference of waves had dis 

 tinctly occurred to Hooke, Newton had failed to see how 

 the periodic phenomena of light could be connected with 

 the periodic character of waves. His hypothesis fell be 

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

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

 descend by mathematical deduction to consequences which 

 could be verified or refuted. 



We are always at freedom again to imagine the existence 

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

 provided there are phenomena incapable of explanation 

 from known causes. We may speak of vital force as oc 

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

 than a name for an undefined something giving rise to 

 inexplicable facts, just as the French chemists called Iodine 

 the Substance X, Avhile they were unaware of its real 

 character and place in chemistry ?. Encke was quite 

 justified in speaking of the resisting medium in space so 

 long 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 

 the 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- 



v Paris, Life of Davy, p. 274. 



