20 SCIENCE PROGRESS 



We shall first of all consider the results obtained by Franck, 

 Knipping, and Kruger {Ber. Dentsch. Phys. Ges., 1919, 21, 728). 

 As a result of carefully repeated experiments, these investigators 

 recognise four distinct stages corresponding to four definite 

 changes which the hydrogen molecule can undergo. 



First, there is a weak but appreciable ionisation of the gas 

 at 11-5 ± 0-7 volts. This is ascribed to the ionisation of the 

 molecule thus: H2->H2'^ + 0, In support of this view 

 Franck and his collaborators point out that the ion formed at 

 this potential is of molecular, and not atomic, dimensions, thereby 

 eliminating the possibility of ascribing the effect to H-^H"*" +0, 

 which might be expected to occur at a higher potential. 



Secondly, it has been observed that a resonance potential 

 manifests itself (photo-electrically) at 13-6 ± 0-7 volts. This is 

 ascribed to a non-electrical process, namely the dissociation of 

 the hydrogen molecule into two atoms, one of which is normal, 

 the other contains two quanta. This change (reversed) corre- 

 sponds to the first line of the Lyman series in the far ultra- 

 violet. This potential is written by the authors as (lo-i -{- Q) 

 volts, where Q = 3*53 ± 0-25 volts. This term will be con- 

 sidered later. 



In the third place, a strong ionisation has been observed 

 at 17-1 ± 0-27 volts, which is written as (13-5 + ^) volts. This 

 is regarded as the ionisation potential of the hydrogen molecule 

 into an atom, a nucleus, and an electron, thus : H2-»H +H"^-f- 0. 



Finally, a new ionisation stage has been observed at the 

 very high voltage 30-4 i 0-5, which is written as (2 x 13-5 -f Q) 

 volts, and is regarded as corresponding to the most violent change 

 of which the neutral hydrogen molecule is capable, namely 

 its ionisation into two nuclei and two electrons, thus : 



H2->2H+ + 20. 



The quantity Q (3-53 volts), which seems to occur in at 

 least three of the observed stages, is ascribed to the dissociation 

 of the hydrogen molecule into neutral atoms. As evidence of 

 the general correctness of this view, it is pointed out that 

 3*53 volts would correspond to 81,300 cals. per gram-molecule 

 of hydrogen, a quantity which agrees fairly well with the heat 

 of dissociation of hydrogen, 84,000 cals., as observed by Lang- 

 muir. This resonance potential, 3-53 volts, was not observed 

 directly, though presumably it was looked for. The photo- 

 electric effect produced by it was evidently too weak to manifest 

 itself under the experimental conditions employed. It would 

 obviously be of the first importance to determine its position 

 accurately and directly. 



It may be pointed out that by subtracting stage 3 from 

 stage 4 we obtain the value i 3-3 volts as the ionisation potential 

 of the hydrogen atom, viz. H -^ H^ + 0. 



