630 FRAGMENTS OF SCIENCE. 



transverse to the direction of propagation. The example 

 of sound was at hand, which was a case of longitudinal 

 vibration. Now the substitution of transverse for longi- 

 tudinal vibrations in the case of light involved a radical 

 change of conception as to the mechanical properties of the 

 luminiferous medium. But though this change went so 

 far as to fill space with a substance possessing the proper- 

 ties of a solid, rather than those of a gas, the change was 

 accepted, because the newly discovered facts imperatively 

 demanded it. Following Mr. Martineau's exa tuple, 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 undertakings 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 implies that not only planets of 

 enormous weight, but comets with hardly any weight at all, 

 fly through your hypothetical solid without perceptible 

 loss of motion." This would sound very cogent, but it 

 would be very vain. Equally vain, in my opinion, is Mr. 

 Martineau's contention that we are not justified in modify- 

 ing, in accordance with advancing knowledge, our notions 

 of matter. 



Before parting from Professor Knight, let me commend 

 his courage as well as his insight. We have heard much 

 of late of the peril to morality involved in the decay of 

 religious belief. What Mr. Knight says under this head is 

 worthy of all respect and attention. ' I admit/' he writes, 

 "that were it proved that the moral faculty was derived as 

 well as developed, its present decisions would not be 

 invalidated. The child of experience has a father whose 

 teachings are grave, peremptory, and august; and an 

 earthborn rule may be as stringent as any derived from a 

 celestial source. It does not even follow that a belief in 

 the material origin of spiritual existence, accompanied by 

 a corresponding decay of belief in immortality, must 

 necessarily lead to a relaxation of the moral fiber of the 



