36 



Supplement to '' Nature'' July 7, 1923 



of spectra. Thus the similarity between X-ray 

 spectra and the spectra emitted during the binding 

 of a single electron to a nucleus may be simply inter- 

 preted from the fact that the transitions between 

 stationary states with which we are concerned in 

 X-ray spectra are accompanied by changes in the 

 motion of an electron in the inner part of the atom, 

 where the influence of the attraction of the nucleus 

 is very great compared with the repulsive forces of 

 the other electrons. 



The relations between other properties of the elements 

 are of a much more complicated character, which 

 originates in the fact that we have to do with processes 

 concerning the motion of the electrons in the outer 

 part of the atom, where the forces that the electrons 



30 -W ^O 60 

 /Htvm/c Numbers 



Fig. 4. 



70 



exert on one another are of the same order of magnitude 

 as the attraction towards the nucleus, and where, 

 therefore, the details of the interaction of the electrons 

 play an important part. A characteristic example 

 of such a case is afforded by the spatial extension 

 of the atoms of the elements. Lothar Meyer himself 

 directed attention to the characteristic periodic change 

 exhibited by the ratio of the atomic weight to the 

 density, the so-called atomic volume, of the elements 

 in the natural system. An idea of these facts is given 

 by Fig. 4, in which the atomic volume is represented 

 as a function of the atomic number. A greater 

 difference between this and the previous figure could 

 scarcely be imagined. While the X-ray spectra 

 vary uniformly with the atomic number, the atomic 

 volumes show a characteristic periodic change which 

 corresponds exactly to the change in the chemical 

 properties of the elements. 



Ordinary optical spectra behave in an analogous 

 way. In spite of the dissimilarity between these 

 spectra, Rydberg succeeded in tracing a certain 

 general relationship between the hydrogen spec- 

 trum and other spectra. Even though the spectral 

 lines of the elements with higher atomic number 

 appear as combinations of a more complicated manifold 

 of spectral terras which is not so simply co-ordinated 

 with a series of whole numbers, still the spectral terms 

 can be arranged in series each of which shows a strong 

 similarity to the series of terms in the hydrogen 

 spectrum. This similarity appears in the fact that 

 the terms in each series can, as Rydberg pointed out, 

 be very accurately represented by the formula K/(w 4- a)^^ 

 where K is the same constant that occurs in the 

 hydrogen spectrum, often called the 

 Rydberg constant, while n is the 

 term number, and a a constant 

 which is different for the different 

 series. 



This relationship with the hydro- 

 gen spectrum leads us immediately 

 to regard these spectra as the last 

 step of a process whereby the neutral 

 atom is built up by the capture and 

 binding of electrons to the nucleus, 

 one by one. In fact, it is clear that 

 the last electron captured, so long as 

 it is in that stage of the binding 

 process in which its orbit is still 

 large compared to the orbits of the 

 previously bound electrons, will be 

 subjected to a force from the 

 nucleus and these electrons, that 

 differs but little from the force with 

 which the electron in the hydrogen 

 atom is attracted towards the nucleus 

 while it is moving in an orbit of corresponding 

 dimensions. 



The spectra so far considered, for which Rydberg's 

 laws hold, are excited by means of electric dis- 

 charge under ordinary conditions and are often called 

 arc spectra. The elements emit also another type 

 of spectrum, the so-called spark spectra, when they 

 are subjected to an extremely powerful discharge. 

 Hitherto it was impossible to disentangle the spark 

 spectra in the same way as the arc spectra. Shortly 

 after the above view on the origin of arc spectra 

 was brought forward, however. Fowler found (1914) 

 that an empirical expression for the spark spectrum 

 lines could be established which corresponds exactly 

 to Rydberg's laws with the single difference that the 

 constant K is replaced by a constant four times as 

 large. Since, as we have seen, the constant that 

 appears in the spectrum sent out during the binding 



SO 



90 



