398 FRAGMENTS OF SCIENCE 



which passed muster in the infancy of knowledge, but 

 which are wholly incompatible with our present enlighten- 

 ment. Mr. Martineau, I think, errs when he seeks to 

 hold me to views enunciated by "Democritus and the 

 mathematicians." That definitions should change as 

 knowledge advances is in accordance both with sound 

 sense and scientific practice. When, for example, the 

 undulatory theory was started, it was not imagined that 

 the vibrations of light could be transverse to the direc- 

 tion of propagation. The example of sound was at hand, 

 which was a case of longitudinal vibration. Now the 

 substitution of transverse for longitudinal vibrations in 

 the case of light involved a radical change of conception 

 as to the mechanical properties of the luminiferous me- 

 dium. But though this change went so far as to fill space 

 with a substance, possessing the properties of a solid, 

 rather than those of a gas, the change was accepted, be- 

 cause the newly discovered facts imperatively demanded 

 it. Following Mr. Martineau's example, the opponent of 

 the undulatory theory might effectually twit the holder 

 of it on his change of front. "This ether of yours," he 

 might say, "alters its style with every change of service. 

 Starting as a beggar, with scarce a rag of 'property' to 

 cover its bones, it turns up as a prince when large under- 

 takings are wanted. You had some show of reason when, 

 with the case of sound before you, you assumed your ether 

 to be a gas in the last extremity of attenuation. But now 

 that new service is rendered necessary by new facts, you 

 drop the beggar's rags, and accomplish an undertaking, 

 great and princely enough in all conscience; for it im- 

 plies that not only planets of enormous weight, but comets 

 with hardly any weight at all, fly through your hypothet- 



