THE USE OF HYPOTHESIS. 143 



The Second Requisite Consistency with established 

 Laivs of Nature. 



In the second place an hypothesis must not be contra- 

 dictory to what we believe to be true concerning Nature. 

 It must not involve self-inconsistency which is opposed to 

 the highest and simplest laws, namely, those of Logic. 

 Neither ought it to be irreconcileable with the simple 

 laws of motion, of gravity, of the conservation of energy, 

 or any parts of physical science which we consider to be 

 established beyond reasonable doubt. Not that we are 

 absolutely forbidden to adopt such an hypothesis, but if 

 we do so we must be prepared to disprove some of the 

 best demonstrated truths in the possession of mankind. 

 The fact that conflict exists means that the conse- 

 quences of the theory are not verified if previous dis- 

 coveries are correct, and we must therefore show that 

 previous discoveries are incorrect before we can verify 

 our theory. 



An hypothesis will be exceedingly improbable, not to 

 say invalid, if it supposes a substance or agent to act in a 

 manner unknown in other cases; for it then fails to be 

 verified in our knowledge of that substance or agent. 

 Several physicists, especially Euler and Grove, have sup- 

 posed that we might dispense with any ethereal basis of 

 light, and infer from the interstellar passage of rays that 

 there was some kind of rare gas occupying space. But if 

 so, that gas must be excessively rare, as we may infer 

 from the apparent absence of an atmosphere around the 

 moon, and from many other facts and laws known to us 

 concerning gases and the atmosphere ; and yet at the same 

 jime it must possess an elastic force at least a billion 

 jfenes as great as atmospheric air at the earth's surface, in 

 order to account for the extreme rapidity of the light 



