THE LUMINTPEROUS ETHEK. 99 



a body moving through it is derived from what we observe in 

 the case of sohds moving through fluids, hquid or gaseous, as 

 the case may be. In ordinary cases of resistance, the main 

 representative of the work apparently lost in propelling the 

 solid is in the hrst instance the molar kinetic energy of the 

 trail of eddies in the wake. The formation of these eddies is, 

 however, an indirect effect of the internal friction, or if we 

 prefer the term viscosity, of the fluid. Now the viscosity of 

 gases has been explained on the kinetic theory of gases, and 

 in the case of a liquid we cannot well doubt that it is con- 

 nected with the constitution of the substance as not being 

 absolutely continuous but molecular. But if the ether be 

 either non-molecular, or molecular in some totally different 

 sense from ponderable matter, we cannot with safety infer 

 that the motion of a solid through it necessarily implies 

 resistance. 



The luminiferous ether touches on another mysterious 

 agent, the nature of whichis unknown, although its laws are in 

 many respects known, and it is applied to the every day wants 

 of life, and its applications are even regulated by Acts of 

 Parliament ; I allude to Electricity. I said that the nature of 

 electricity is unknown. More than forty years ago I was sit- 

 ting at dinner beside the illustrious Faraday, and I said to 

 him that I thought a great step would have been made if we 

 could say of electricity something analogous to what we say 

 of light, when we affirm that light consists of undulations ; 

 and he said to me that he thought we were a long way off 

 that at present. But, as I said, relations have recently been 

 discovered between light and electricity which lead us to 

 believe that the latter is most closely connected with the 

 luminiferous ether. 



Clark Maxwell showed that the ratio of two electrical con- 

 sta)its which are capable of being determined by laboratory 

 experiments, and which are of such a nature that that ratio 

 expresses a velocity, agrees with remarkable accuracy with 

 the known velocity of light. This formed the starting point 

 of the electro-magnetic theory of light which is so closelv 

 associated with the name of Maxwell. 



According to this idea, light may be looked on as the pro- 

 pagation of an electro-magnetic disturbance, whatever the 

 appropriate idea of such a thing may actually be. The theory 

 has quite recently received remarkable confirmation by the 

 investigations of Hertz, who has shown that what are incon- 

 testably electro-magnetic disturbances, and are investigated 



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