Radiation and -Ionization of Helium. 751 



The chief importance of Franck and Knipping's later paper 

 lies in the fact that their conclusions afford a confirmation of 

 Franck and Reiche's view that in the normal state of the 

 helium atom the two electron orbits are crossed. Their curves 

 show a first marked bend for pure helium at 2L'2 volts, 

 while, with the impure gas, marked bends occur at both 

 20'4 volts and 21*2 volts. These results are interpreted as 

 shoving that the detection of radiation at 204 volts depends 

 upon the presence in the helium of a small amount of 

 impurity. At 20'4 volts an inelastic collision is supposed to 

 occur, by which the outer electron of the normal helium 

 atom is removed to the first outer orbit coplanar with the 

 orbit of the inner electron (the transition being rendered pos- 

 sible because the strong electric fields generated momentarily 

 modify the limitations of the selection principle), and the 

 absence of a marked bend in ihe curves at 20"4 volts in pure 

 helium is taken to indicate that the abnormal atoms so pro- 

 duced do not revert to the normal condition. As has been 

 pointed out, it is not possible, in accordance with the selection 

 principle of Bohr, Sommerfeld and Rubinowitz, for reversible 

 transitions to occur between the crossed and the coplanar 

 systems of orbits, so that the absence of radiation at 

 20*4 volts in pure helium would confirm the view that in the 

 normal atom the orbit of the outer electron is not of the 

 same system as that to which it is removed by the absorption 

 of 20*4 volts energy, and that the orbits are therefore crossed 

 in the normal atom. 



The detection of a sharp bend at 20'4 volts in the curves 

 of Franck and Knipping in impure gas is explained by them 

 on the view that in the metastable coplanar condition the 

 constitution of helium resembles that of the alkali metals, 

 and that, like them, it is capable of forming halogen com- 

 pounds and an oxide. In the presence of impurity the 

 metastable helium atoms enter into short-lived combinations 

 with impurity atoms and, by the formation of these compounds, 

 external disturbances, sufficient to modify the limitations 

 of the selection principle, are generated and the coplanar 

 helium atoms revert to the normal state with an emission of 

 radiation. 



Franck and Reiche's model of the normal helium atom 

 has recently been criticised by Kemble * on the grounds 

 that their identification of the normal orbit of the outer 

 electron with the (1, S) orbit of the sharp series of crossed 

 helium does not lead to quite the correct energy for the 

 normal helium atom, and that it is not in accord with the 

 * E. Kemble, Phil. Mag. xlii. p. 123 (1921 ). 



