4 Prof. Oliver Lodge on 



of the doubter and say to him, Disprove it if you can ; and so 

 we practically say for all our axioms, and for all laws which 

 are so simple and fundamental as to be hardly distinguishable 

 from axioms. 



Experiments are often made or adduced in support of a 

 law as if they were part of its foundation : thus Newton tried 

 experiments on impact before stating his third law, but the 

 experiments did not really prove it with accuracy even for the 

 particular case examined. All they could show was that there 

 was nothing obviously wrong with it. He saw no reason for 

 supposing it wrong, and so after consideration stated it as an 

 axiom, to be hereafter challenged and found inaccurate if so 

 the progress of experience turned out. 



I should say that an axiom or fundamental physical law is 

 a simple statement, suggested by familiar or easily ascertained 

 facts, probable in itself, readily grasped, and not disproved or 

 apparently liable to disproof throughout a long course of 

 experience. 



If a statement is capable of exact examination and verifica- 

 tion, either by reasoning or by experiment, it is called a law, 

 but not a fundamental law ; i. e. it is no longer part of the 

 foundation, it is supported on something else. If it has no 

 support, except the absence of evidence against it, it is an 

 axiom. Far be it from me to decry the use of experiments 

 of verification. The necessity for them whenever feasible is 

 conspicuous and universally admitted, and much ingenuity 

 may be usefully spent upon them ; but I do say that in time 

 a theory can become established by processes other than direct 

 experimental verification ; and in fact that really valid and 

 flawless experimental verification is frequently an impos- 

 sibility. 



An instructive example of the legitimate strength of a 

 theory, even when opposed by apparent facts, is contained in 

 an article by Lord Rayleigh in the Philosophical Magazine 

 for March 1889, " On the History of the Doctrine of Kadiant 

 Energy." 



It appears that W. Herschel conceived the idea that the 

 radiation which excites the sensation of light and the radia- 

 tion which produces heat in black bodies are essentially 

 different ; and this view, which was contrary to his original 

 intuition, was based upon a crucial prismatic experiment, 

 made for the purpose of ascertaining " whether the heat of 

 the red rays is occasioned by the light of those rays " or not. 

 A definite question, answered by experiment in the negative* 

 On this Lord Ilayleigh remarks : — 



" I am disposed to think that it was this erroneous conclu- 



