SOME CONTllMl'OR.IRY . ;/'C. ;.V(/:S IX fliySlCS in 2S0 



()(>triili.ils .111(1 I'xcili'il ltr(Hiriuifs. Tin- (iiii'stion is ot IukIi impnrl- 

 .iiu-e, not simply hivaiisc we arc inliTested to know wlifthcr soini- of 

 tin- excited rays really lie in the hitherto unpenetrated ran^e, Imt 

 primarily because excitation and emission are among tlie fimd.imcntal 

 qualities of atoms. Kxcitation-potentials exceeding lOOO volts 

 generally produce rays of which the wave-lengths are less than 12.^ 

 and can Ik' measuretl with the crystal spectrograph, so that a rule o"- 

 law can l>e deduced from the two sets of measurements. Kxcitation- 

 potentials inferior to 2.t volts generally produce ra\'s of which the 

 wa\e-lengths are greater than oOO.A and can be measured with optical 

 apparatus, and again a law can be deduced from the two sets of data. 

 But the law is not the same in the two cases; this is because excita- 

 tion, in the former case, consists in displacing a deep-lying electron, 

 while in the latter case it consists in a displacement of the \'alence- 

 electron. We are forced to the disconcerting conclusions that excita- 

 tion-potentials between 1000 volts and 2.5 volts inxoKc electrons of 

 an intermediate type, and that the still-unverifiable law connecting 

 them with the frequencies of their excited rays is not identical with 

 cither of the laws in the accessible regions of the spectrum. 



The law for excitation-potentials invoking displacements of the 

 valence-electron is twofold. Kach atom has at least tw-o such excita- 

 tion-potentials. One of them is its ionizing-potential. When the 

 accelerating-voltage of an electron-stream playing against a multi- 

 tude of free atoms forming a gas is raised just past the value T',- at 

 which an individual electron has just enough energy to remo\-e the 

 valence-electron of an atom, there is an outburst of radiation. This 

 comprises rays of many frequencies — probably all those which we 

 have called valence-electron rays — and they are emitted as the 

 valence-electrons descend step-by-step along their ladders of orbits. 

 All these frequencies conform to the relation 

 (1) hy<eVi. 



The other excitation-potential is the resoininct' potential of the atom 

 (there may be more than one of these "'). When the accelerating- 

 equal to e\',h. The heterogeneous lieams used by Holwcck in the experiments 

 previously cited consisted chiefly, if not entirely, of this continuous spectrum. .■\11 the 

 excitation-potentials mentioned in these (Kiges, however, relate to individual rays or 

 groups of individual rays characteristic of atoms. 



"This question is still incompletely solvefl, in spite of much labor. .At one time 

 it was supjiosed that the valence-electron could Ik- raised either altogether out of the 

 atom, or else to the dee|)est-lying of the transient-sojourn orbits lor to either of the 

 two deepest-lying orbits, if there arc two complete families of orbits such as the 

 mercury atom possesses); but not to any of the other transient-sojourn or "virtual" 

 orbits. This restriction would apply only to displacements caused by impinging 

 electrons; quanta of appropriate frequencies can lift the valence-electron to any of 



