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X. A Critical Study of Spectral Series.—Part IV. The Structure of 
Spark Spectra. 
By W. M. Hicks, F.R.S. 
Received May 18,—Read June 1, 1916. 
[Plates 4-18.] 
The spectrum of an element produced by the electric spark is in general funda¬ 
mentally different from that produced by the arc. The latter is marked by the 
predominance of groups of lines forming series associated with one another, and by 
lines related to these series in definite ways. In the former very few series have been 
discovered, the lines are very much more numerous, and the great majority are weak. 
Both kinds vary very much according to the way in which they are produced, and 
this is specially the case with spectra produced by the spark. Even when they are 
produced by similar methods by different observers—as, for instance, by Eder and 
Valenta, and by Exner and Haschek —there will be found a large .number in one 
not included in the other and vice versd. It is even questionable whether it is 
possible to draw a distinct and definite line of demarcation between the two, e.g., in 
the cases of Cu and Ba, to take two instances ; the arc spectra—with very numerous 
lines—in many respects have analogies with those of the spark. But very little is 
known as to the structure of the latter, beyond the fact that in some of them sets of 
lines with the same frequency differences are met with. The present communication is 
an attempt to throw some light on this question. 
The material at disposal is so vast that it is necessary to limit the discussion to a 
few elements, and even in them to restrict it to certain relations only. The elements 
selected are silver and gold. It had originally been intended to include also copper 
and barium as illustrating the transition from one to the other kind of spectra. But 
their inclusion would have rendered the present communication unduly long, whilst 
their omission enables us to confine the discussion to the elaboration of a single 
principle. The reason for the selection is that the author has had occasion to study 
with special care for other purposes the spectra of Ag and Ba, whilst those of Au and 
Cu—belonging to the same group as Ag—were specially investigated to see if the 
results afforded by Ag were supported by them. That of Cu was also interesting, as 
in the arc there are a very large number of doublets and triplets discovered by 
Rydberg; indeed the origins of some of the frequency differences observed were first 
VOL. CCXVII.-A 558. 3 E [Published October 23, 1917. 
