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the centres of the light emission on electric discharges also for simple 
gases. In consideration of previous researches on collisions of electrons 
with gas molecules Frank and Hertz defined a theoretical view about 
mixtures of gases more closely, assuming that in electro-positive and in 
the rare gases the electrons collide elastically with the gas molecules, 
so long as the energy does not exceed that amount that corresponds 
with the ionisation-potential. They derive that with mixtures 
of gases the light-emission takes place preponderantly through the 
gas with the slightest ionisation-potential. Indeed, in many cases 
this theory appears qualitatively to harmonize with experience. 
Already in our Thesis we drew attention to the deviation in mixtures 
of argon and mercury, and in the Zeitschrift für wissenschaftliche 
Photographie (18, 43, (18)) we pointed out that our experimental result 
in the field of spectral intensity contirms the opinion that for mercury the 
determination of the potential of ionisation at 10 Volts is preferable to 
the value given earlier by others (5 Volts). We have, however, also drawn 
attention in our ‘Thesis’ to objections to the theory of Frank and 
Hertz, among other things on the ground of the fact that light-emis- 
sion can also take place without ionisation. Indeed, Frank and Hertz 
themselves have published a modification of the theory in question in 
connection with Bour’s results (Phys. Zeitschr. 20, 132 (19). 
The necessity of this modification follows particularly from the 
fact that also with values lying below the ionisation-potential 
electric discharges through gases can take place. Accordingly FRANK 
and Hertz abandoned their view of the perfectly elastic collision, 
and like Cuinp, they assumed in agreement with Bonr’s theory, that 
when an atom collides with an electron, and the energy of the 
latter is sufficient, one of the electrons of the atom can pass from 
its path to one lying more on the outside. It is now conceivable 
that a return to the normal path takes place with light-emission, 
but also that before this happens, the injured atom again collides 
with another electron, so that a further change of path takes place 
ete. etc, till at last the partial ionisation can have changed into a 
complete ionisation. 
c. So it is seen here that the optical phenomena compel us to 
assume dislocated atoms. Even in case of a rare gas multifarious 
particles will occur in the path of discharge according as the disloca- 
tion of the atoms takes place to a greater or less degree. When 
we have to do with molecules, i.e. with atom complexes, the ques- 
tion will be much more complicated. For of each of the atoms in 
the molecule one of the electrons in the “outer path” of the atom 
in question can be in an abnormal condition. To this complicated 
