Radiation and the Structure of the Atom, 411 



with the assumption that this voltage corresponds only to the 

 transition from the normal state to some other stationary 

 state of the neutral atom. On the present theory we should 

 expect that the value for the energy necessary to remove an 

 electron from the mercury atom could be calculated from 

 the limit of the single line series of Paschen, 1850, 1403 r 

 1269 *. For since mercury vapour absorbs light of wave- 

 length 1850 f, the lines of this series as well as the line 2536 

 must correspond to a transition from the normal state of the 

 atom to other stationary states of the neutral atom (see I. 

 p. 16). Such a calculation gives 10*5 volts for the ionization 

 potential instead of 4*9 volts J. If the above considerations 

 are correct it will be seen that Franck and Hertz's measure- 

 ments give very strong support to the theory considered in 

 this paper. If, on the other hand, the ionization potential 

 of mercury should prove to be as low as assumed by Franck 

 and Hertz, it would constitute a serious difficulty for the 

 above interpretation of the Rydberg constant, at any rate 

 for the mercury spectrum, since this spectrum contains lines 

 of greater, frequency than the line 2536. 



It will be remarked that it is assumed that all the spectra 

 considered in this section are essentially connected with the 

 displacement of a single electron. This assumption — which 

 is in contrast to the assumptions used by Nicholson in his 

 criticism of the present theory — does not only seem supported 

 by the measurements of the energy necessary to produce the 

 spectra, but it is also strongly advocated by general reasons 

 if we base our considerations on the assumption of stationary 

 states. Thus it may happen that the atom loses several 

 electrons by a violent impact, but the probability that the 

 electrons will be removed to exactly the same distance from 

 the nucleus or will fall back into the atom again at exactly 

 the same time would appear to be very small. For molecules, 

 i. e. systems containing more than one nucleus, we have 

 further to take into consideration that if the greater part of 

 the electrons are removed there is nothing to keep the nuclei 

 together, and that we must assume that the molecules in such 

 cases will split up into single atoms (com p. III. p. 858). 



* Paschen, Ann. d. Phr/s. xxxv. p. 860 (1911). 



t Stark, Ann. d. Phys. xlii. p. 239 (1913). 



X This value is of the same order of magnitude as the value 

 12*6 volts recently found by McLennan and Henderson (Proe. Roy. 

 Soc. A. xci. p. 485, 1915) to he the minimum voltage necessary to 

 pruduce the usual mercury spectrum. The interesting observations 

 of single-lined spectra of zinc and cadmium given in their paper are 

 analogous to Franck and Hertz's results for mercury, and similar 

 ■considerations may therefore possibly also hold for them/ 



