io Dr. Beatty on 



are waves propagated through the ether. When such waves, 

 which we may think of as rapidly oscillating pulses, pass over 

 electrons which can be set into vibration about their steady 

 positions, then these electrons may be made to vibrate strongly if 

 their rate of oscillation is close to that of the waves themselves. 

 Thus light waves are able to cause vibrations of electrons in an 

 outer ring, and consequently an emission of light from the atoms 

 concerned. This is the phenomenon of fluorescence. When 

 iodine vapour is present in a glass bulb, no other vapour or gas 

 being admitted, and a beam of white light is sent through the bulb, 

 the iodine vapour will glow with a bright greenish-yellow light 

 ' along the track of the beam. In a similar manner X-rays which 

 have a much shorter wave-length will affect the inner rings of 

 electrons and cause the atom to give out X-rays peculiar to itself: 

 these are known as characteristic X-rays. 



Thus atoms, in addition to giving out visible spectra, also 

 emit X-ray spectra : these are remarkable for their great simplicity, 

 consisting only of two or three bright lines. Further, the element 

 of periodicity is absent from the inner rings. As we go along the 

 elements to higher and higher atomic weights, the successive 

 atomic X-ray spectra contain shorter and shorter wave-lengths. 

 That is, the inner rings are more and more rigid as the atomic 

 weight increases. 



X-ray spectra fall into two series at least, and each series may 

 be attributed to a separate electronic ring. Hence we deduce 

 that many elements, certainly those of greater atomic weight than 

 calcium, have at least two inner rings of electrons. The thorough 

 investigation of the behaviour of atoms under the influence of 

 X-rays is helping the physicist a long way towards the complete 

 specification of atomic structure. 



A second powerful agent in examining the central regions of 

 the atom is supplied by the particles and radiations which radium 

 ernits continuously. The a-particle from radio-active substances 

 is an atom of helium, which, at the moment when it is fired out is 

 deprived of both its electrons, and so consists merely of the positive 

 nucleus of helium travelling with enormous speed. The mass and 



