260 
PROF, A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
The varying facility with which enhanced lines can be produced in the spectra of 
different elements would seem to depend upon the intensities of the forces which bind 
the electrons to the nuclei. In some elements, such as lithium, no enhanced lines at 
all have been observed. The lithium atom is credited with three electrons, and Bohr’s 
calculations indicate that while the outer electron is lightly bound, the inner two are 
very strongly bound as compared with the electron in an atom of hydrogen, and even 
more rigidly than the two electrons in a helium atom. Theory and experiment are 
thus in agreement, and the production of enhanced lines of lithium would appear to 
require more powerful spark discharges than those hitherto employed. 
In the case of elements like calcium, which give enhanced lines of Class I., it may 
be supposed that one electron is lightly bound, so that even a flame may effect its 
removal, while a second is removable by the moderate increase in the energy of excitation 
when passing from the flame to the arc. The further increase in the intensities of the 
enhanced lines on passing to the spark may be attributed to the relatively greater 
number of atoms from which two electrons are simultaneously detached. In elements 
like iron, giving enhanced lines of Class II., the conditions may be supposed similar, 
except that the second electron is removed with somewhat greater difficulty. 
Enhanced lines like Mg 4481 require a modified explanation. Although the series 
to which this line belongs is produced only under spark conditions (or their equivalents 
in modified arcs) it is important to bear in mind that the related system of Wide 
doublets, consisting of enhanced lines of Class I., is developed in the ordinary arc. 
Since the term 4N appears in the formulae for both systems, Bohr’s theory would lead 
to the supposition that in this case the greater energy of the spark does not result in 
the separation of a third electron from any of the atoms but merely produces some 
change in the configuration of the atomic systems. The experimental data are not 
sufficiently complete to justify the conclusion that such an explanation would be 
applicable to all enhanced lines of Class III. 
If, by any means, three electrons could be separated from the nucleus, the appearance 
of lines for which the series constant was raised to 9N would be expected, if Bohr’s 
theory is a safe guide. Great energy would apparently be needed to bring about this 
result, and no such series have yet been recognised. A sufficient increase in the energy 
of excitation would presumably yield series requiring still greater multiples of the 
constant N in the formulae. 
It will be observed that Bohr’s theory gives a more definite conception to Lockyer’s 
earlier view that the varying spectra given by the same substance represent the 
vibrations of different “molecular groupings,” such groupings being simplified by 
successive dissociations brought about by increase of temperature or electrical 
excitation. 
It should be noted that some of the conclusions drawn by Stark from his experi¬ 
ments on canal-ray spectra are inconsistent with the views of Bohr. In the case of 
helium, Stark has found that the Doppler displacements are smaller for the doublet 
