xxni.] THE USE OF HYPOTHESIS. 523 



consists of particles of matter is even more absurd than to 

 reinstate the corpuscular theory of light. A far closer 

 analogy exists between electricity and light undulations, 

 which are about equally rapid in propagation. We shall 

 probably continue for a long time to talk of the electric 

 fluid, but there can be no doubt that this expression 

 represents merely a phase of molecular motion, a wave of 

 disturbance. The invalidity of these fluid theories is 

 shown moreover in the fact that they have not led to the 

 invention of a single new experiment. 



Among these merely descriptive hypotheses I should 

 place Newton's theory of Fits of Easy Reflection and 

 Refraction. That theory- did not do more than describe 

 what took place. It involved no analogy to other pheno- 

 mena of nature, for Newton could not point to any other 

 substance which went through these extraordinary fits. 

 We now know that the true analogy would have been 

 waves of sound, of which Newton had acquired in other 

 respects so complete a comprehension. But though the 

 notion of interference of waves had distinctly occurred to 

 Hooke, Newton failed to see how the periodic phenomena 

 of light could be connected with the periodic character of 

 waves. His hypothesis fell because 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 at freedom to imagine the existence of a new 

 agent, and to give it an appropriate name, provided there 

 are phenomena incapable of explanation from known 

 causes. We may speak of vital force as occasioning 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 Sub- 

 stance X, so long as they were unaware of its real cha- 

 racter and place in chemistry. 1 En eke was quite justified 

 in speaking of the resisting medium in space so long as the 

 retardation of his comet could not be otherwise accounted 

 for. But such hypotheses will do much harm whenever 

 they divert us from attempts to reconcile the facts with 



1 Paris, Life of Davy, p. 274. 



