Radiation and Ionization of Helium . 771 



If in pure helium, He 2 molecules result from the absorption 

 by normal helium atoms of 20'4 volts energy (these mole- 

 cules being unaffected by impurity), it is possible that the 

 ionization which we have detected in the presence of 

 21*2 volts radiation is due to the ionization of these He 2 

 molecules and not to the ionization of the abnormal atoms as 

 such. Our experiments have shown that the 20*4 volts 

 abnormal atoms are able to revert directly to the normal con- 

 dition, and the fact that they appear to do so less readily 

 than the abnormal atoms resulting from the absorption of 

 2T2 volts energy, may be due to the fact that when two 

 20*4: volts atoms encounter one another, the formation of 

 He 2 is possible. Further experiments are needed before any 

 definite decision on this point can be given. 



Conclusion. 



The experiments described in this paper have shown that 

 in pure helium inelastic collisions of electrons with helium 

 atoms take place at 20°4 volts and at 21'2 volts and that both 

 types of inelastic collision give rise to resonance radiation. 

 Reversible transitions therefore occur between the normal 

 orbit of the outer of the two electrons in the helium atom, 

 and the orbits to which this electron is removed at 20'4 volts 

 and at 24*2 volts respectively, i. e. orbits of the coplanar and 

 of the crossed systems respectively. Even if the detection 

 of radiation at both critical points could bo explained by 

 the presence of a small trace of impurity } which enabled the 

 limitations of the selection principle to be modified, the 

 demonstration that both types of radiation are resonance 

 radiations in the strictest sense of the word, i. e. that both 

 types of radiation can be absorbed and re-emitted by other 

 atoms, cannot be explained away in this manner, for the 

 absorption of each type of radiation by normal helium atoms 

 means that the production of 20*4 volts abnormal atoms, and 

 21'2 volls abnormal atoms, occurs when there are no strong- 

 electric fields to affect the limitations of the selection 

 principle. Thus, whether the normal orbit of the outer 

 electron is of the crossed or of the coplanar system, it is 

 evident that a reversible transition from the one system to 

 the other occurs either at 20'4 volts or at 21*2 volts. Our 

 experiments thus show that the limitations of the selection 

 principle of Bohr, Spmmerfeld. and Rubinowitz do not apply 

 rigidly to the fundamental displacements of the outer electron 

 of the helium atom. In this connexion it must be mentioned 



