254 
PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
fig. 1, in which the doublet separations have been arbitrarily increased for greater 
clearness. 
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Fig. 1. Enhanced-line series of magnesium. 
§ 11. General Comparison of Mg, Ca, Sr, and Ba. 
One of the results of the investigation is to indicate that Mg behaves differently 
from Ca, Sr, and Ba in the spark. All are alike in having series of Wide doublets 
with separations related to the atomic weights, but the D doublet with negative 
frequency which occurs in the other three elements is lacking in Mg. While Ca, Sr, 
and Ba have strong Fundamental series with limits derived from the negative D 
doublet, the corresponding F series of Mg, like the negative term from which it should 
be derived, does not appear to exist. In Mg the equivalent of the F series of the 
other three elements is replaced by the whole system of “ F.P.” doublets (including 
the “ 4481 ” series as Fundamental), which has no analogue in the other three elements 
so far as is known at present. The “ F.P.” system itself is closely related to the family 
of Wide doublets, but although both consist of enhanced lines they do not necessarily 
co-exist ; the wider system occurring in the ordinary arc, while the closer requires the 
conditions of the spark or its equivalent, the negative pole of an arc in vacuo. 
The “ 4481 ” series is therefore established as a Fundamental series, but the system 
of which it forms a part is in some respects exceptional. 
It is important to observe that no numerical relations between the enhanced and 
arc line series occurring in the spectra of any of the four elements have been 
recognised. 
§ 12. The Spectra of Hydrogen and Helium. 
The now demonstrated existence of series having 4N for the “ universal ” constant 
has an interesting bearing on the question of the origin of the special lines produced 
