A HISTORY OF SCIENCE 



Then, about the middle of the century, came that final 

 revolution of thought regarding the nature of energy 

 which we have already outlined in the preceding chap- 

 ter, and with that the case for ether was considered to 

 be fully established. The idea that energy is merely a 

 ' ' mode of motion " (to adopt Tyndall's familiar phrase) , 

 combined with the universal rejection of the notion of 

 action at a distance, made the acceptance of a plenum 

 throughout space a necessity of thought so, at any 

 rate, it has seemed to most physicists of recent dec- 

 ades. The proof that all known forms of radiant en- 

 ergy move through space at the same rate of speed is 

 regarded as practically a demonstration that but one 

 plenum one ether is concerned in their transmission. 

 It has, indeed, been tentatively suggested, by Pro- 

 fessor J. Oliver Lodge, that there may be two ethers, 

 representing the two opposite kinds of electricity, but 

 even the author of this hypothesis would hardly claim 

 for it a high degree of probability. 



The most recent speculations regarding the proper- 

 ties of the ether have departed but little from the early 

 ideas of Young and Fresnel. It is assumed on all sides 

 that the ether is a continuous, incompressible body, 

 possessing rigidity and elasticity. Lord Kelvin has 

 even calculated the probable density of this ether, and 

 its coefficient of rigidity. As might be supposed, it is 

 all but infinitely tenuous as compared with any tangible 

 solid, and its rigidity is but infinitesimal as compared 

 with that of steel. In a word, it combines properties 

 of tangible matter in a way not known in any tangible 

 substance. Therefore we cannot possibly conceive its 

 true condition correctly. The nearest approximation, 



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