280 BELL SYSTEM TECHNICAL JOURNAL 



as —\/h times the energ>-\alue of an additional orbit. If thisorl)it 

 is the permanent home of the valence-electron, the energy required 

 to ionize the atom must be +59.49/; • 10", equivalent to 24.5 ^■olts. 

 When the new lines were discovered, the accepted value was 25.3 

 volts, largely because of a certain measurement by Franck. After 

 the publication of Lyman's discover>% Franck re-examined his method 

 and data and found them compatible with the value 24.5 volts; and 

 \ery recently C. A. Mackay has ascertained that the ionizing-potential 

 of helium is 14.1 \oUs greater than that of mercury, which is quite 

 definitely known to be 10.4. One could hardly desire a better illus- 

 tration of the confluence of measured values of the energies of atoms 

 and measured values of their radiation-frequencies, when both are 

 interpreted according to the contemporary theory of radiation. 



These newly-discovered waves must be the shortest in the spectrinn 

 of helium; the atom cannot emit a ray of wa^■e-k■ngth less than tiie 

 series-limit 504A, calculated by the equation 



/!v = /ic/X =energy-\alue of the deep-lying orbit =59.49- lO'V;. 



The\' are much shorter tJian the waves of the L\inan series of liydrogen, 

 which Bohr's theory, together with the obser\-ed value of ionizing- 

 potential of atomic hydrogen, justify us in declaring to be the shortest 

 waves emitted by that atom. Furthermore, it is almost certain that 

 they are shorter than any waves for which the \alence-electron of 

 any atom is responsible; for the ionizing-potential of helium is greater 

 than any other measured ionizing-potential, and there is no reason to 

 believe that any of the yet unmeasured ones exceed it. Its nearest 

 rivals are the ionizing-potentials of the inert gases which share the 

 last column of the periodic table. The experimentalists have not 

 agreed very well in their estimates of these, although all agree that 

 the values are comparati\ely high. Hertz, the latest to make measure- 

 ments upon neon and argon, gives 21.5 \olts for the first and 15.3 

 volts for the second. Both, therefore, should emit some ra>'s King 

 below 1200A, but above 575A and 8()0A, respectively, and resulting 

 from transitions of the \alence-eIectron. Dcjardin gives 12.7 volts for 

 the ionizing-potential of kr\pton and \ery lately 10.9 Nolts for that of 

 xenon. In the other columns nf ilif periodic table, the \alues of 

 ionizing-potential are pre\ai]ingl\ lower lii.in in tiie ((ijunm of inert 

 gases. The value 10.4 \i)lts (for mercur},) is the highest among 

 the metals; se\eral of the non-metallic elements appear to ha\'e 

 ionizing-potentials between 12 and 17 \-olts, but for some of these 

 it is difficult to tell whether the obser\ed value pertains to the atom 



