790 Prof. A. LI. Hughes on Dissociation of 



Franck, Knipping, and Kriiger * have recently published 

 an account of the ionizing potentials of hydrogen, and they 

 suggest that there are two types of ionization. The first type 

 is ionization, without dissociation, when the electrons have 

 energies in excess of 11 volts. The second type of ionization 

 is accompanied by dissociation. Since 3*6 volts measures 

 the energies required for dissociation and 13'5 volts measures 

 the theoretical energy required for ionizing one atom, there 

 should be an ionizing potential at 3*6 + 13*5 = 17*1 volts, 

 corresponding to dissociation and ionization of one of the 

 atoms. They verified this experimentally. Similarly they 

 found a radiating potential at 13'6 volts corresponding to 

 dissociation combined with radiation from one of the atoms 

 (3'6 + 10 , 2 = 13'8 volts). They also verified the existence of 

 an ionizing potential at 3*6 + 2 X 13*5 = 306 volts, corre- 

 sponding to dissociation and ionization of both atoms. On 

 this theory, then, one would expect dissociation to accompany 

 these types of ionization of, and radiation from, hydrogen. 

 On account of the first type of ionization (11 volts), it is 

 possible that all ionizing collisions are not all dissociating 

 collisions. 



Wendt f has found that if hydrogen is driven through 

 a tube across which an electric discharge is passing, the 

 hydrogen afterwards contains a small amount of triatomic 

 hydrogen H 3 . In his experiments, the pressure was high 

 enough to insure that the mean free path of a molecule was 

 considerably less than the diameter of the tube, so it is 

 natural to infer that the first step in the formation of H 3 is 

 the dissociation of H 2 by electron impacts into atoms H, 

 which in turn unite with molecular hydrogen at the next 

 collision to form H 3 . It is difficult to see any way of 

 accounting for H 3 without the intermediate step of the for- 

 mation of EL Dempster t has found evidence for the view 

 that the formation of H is a necessary step in the formation 

 of Ho. The hydrogen spectrum contains several series 

 (e.g., the Balmer series and the Lyman series), which are, 

 according to Bohr's theory, characteristic of the atom. It 

 thus appears that there are good grounds for believing that 

 atomic hydrogen is formed in the electric discharge, and 

 probably as a result of direct impact of electrons on 

 molecules. 



* Franck, Knipping, and Kriiger, Verh. d. Deutsch. Phys. Ges. xxi. 

 p. 728 (1919). 



t Wendt, Nat, Acad. Sci. Proc. v. p. 518 (1919). 

 % Dempster, Phil. Mag. xxxi. p. 438 (1916). 



