Ou tJic Transparency of tlic EtJicr. 5 



high rates of distortion and its apparently perfect fluidity for 

 motions of distortion of low rates. The existence of such 

 apparently incompatible qualities does not seem so difficult 

 to understand, when a material substance subjected to rates 

 of distortion of far less range than the extreme limits at which 

 these two qualities are observed to exist in the case of the 

 ether appears in the one case like a rigid solid, and in the 

 other like a very mobile fluid. 



Maxwell found that, by rotating a cylinder rapidly in a 

 liquid and passing a ray of polarized light close to its surface, 

 the plane of polarization was altered, proving clearly a state 

 of strain for ordinary liquids when the rate of distortion is 

 sufficently high. Sir William Thomson has also shown how 

 wax or pitch may, in the one case, vibrate like ordinary 

 solids, and, in the other, allow bodies to pass very slowly 

 through them without appreciable resistance. The luminif- 

 erous medium presents similar phenomena. For periods of 

 vibration comparable with those of light, it acts like a very 

 elastic solid. For low rates of distortion like those which 

 the motions of the planets and comets as well as those which 

 the molecules of a gas produce, there is no sensible resistance, 

 and the medium seems to act like a perfect fluid. That this 

 resistance is exceedingly small is shown by the fact that the 

 comets, which are in general of^ extreme tenuity, give no 

 definite indications of a resisting medium in space. 



While the properties of the ethereal medium manifestly 

 transcend those of ordinary matter, yet it seems to fulfil, in 

 the qualities of elasticity and fluidity, the conditions of 

 natural bodies. Very strong analogy to the ether is furnished 

 by viscous substances, and these substances always dissipate 

 more or less rapidly any vibrations to which they are sub- 

 jected, proportionally to the rate of distortion, — at least 

 for small rates. If the ether has a corpuscular structure, — - 

 and it is difficult to conceive of absorption otherwise, — and 

 the analogy in respect to viscosity is extended to it, as well 

 as the analogy in respect to its elasticity and fluidity, there 

 should be a loss or transformation of radiant energy. 



5 



