THE LATEST STAGE 241 



while they exist, so that, by placing the gas in circuit 

 with a battery and an electrometer, Rutherford has 

 shown the effect of each a particle by the sudden throw 

 of the needle of the instrument. Secondly, the ions 

 act as nuclei for the condensation of mist, and, in this 

 way, C. T. R. Wilson has made visible as a line of 

 cloud the track of each particle. 



As we have seen, the revival of the old theory of an 

 all-pervading aether was due to the researches of 

 The Lumini- Young and Fresnel, who re-established 

 ferous ^Ether. the wave theory of light. If light be 

 waves, something to undulate must exist through 

 space. Maxwell's electrical theory demanded a simi- 

 lar something to carry electromagnetic waves, and 

 strengthened greatly the tendency to assume that 

 this something was a subtle, material or quasi- 

 material substance, extended permanently throughout 

 all space, linking together the most distant stars, and 

 capable of being described in mechanical terms. 



It was, however, clear at once that aether must 

 have properties unlike those of any other type of 

 matter known to us. Light-waves oscillate in direc- 

 tions at right angles to the line of sight. Hence the 

 medium which carries them must possess elasticity 

 of shape. In this it must be analogous to a solid. 

 Yet, since it offers no appreciable resistance to the 

 motion of planets, it must be not only fluid but ex- 

 cessively attenuated. Reconciliation between these 

 results was sought in different ways. For instance, 

 a medium was invented rigid to very rapid movements, 

 but offering no resistance to movements compara- 

 tively slow. Others followed one another fast our 



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