THE ETHER AND PONDERABLE MATTER 



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 TyndalPs 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 decades. 

 The proof that all known forms of radiant energy 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, in- 

 deed, been tentatively suggested, by Professor 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 de- 

 gree of probability. 



The most recent speculations regarding the properties 

 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, possess- 

 ing rigidity and elasticity. Lord Kelvin has even cal- 

 culated the probable density of this ether, and its coeffi- 

 cient 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, 

 according to Lord Kelvin, is furnished by a mould of 

 transparent jelly. It is a crude, inaccurate analogy, of 

 course, the density and resistance of jelly in particular 

 being utterly different from those of the ether ; but the 



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