Section III, 1920 [9] Trans. R.S.C. 



On the Absorption and Series Spectra of Lead 

 By Professor J. C. McLennan, F.R.S., and R. V. Zumstein, M.A. 



(Read May Meeting, 1920) 



SYNOPSIS. 



1. Absorption spectra of lead. 



The paper describes experiments which enabled the absorption of non-luminous 

 lead vapour to be determined and it also gives a list of the reversals obtained 

 in a lead-carbon arc. 



2. Series spectra. 



Some suggestions are put forward for grouping into a principal series, two 

 subordinate series and a combination series, certain prominent wavelengths 

 in the spectrum of lead. Considerations are also presented in support of the 

 view that three of the wavelengths in the spectrum of lead constitute members 

 of an enhanced, series. 



I. Introduction 



Up to the present no progress appears to have been made in 

 classifying the wavelengths in the spectrum of lead into series 

 except in the region of the X-ray characteristic radiations. Several 

 observers have pointed out wavelengths in the spectrum of this 

 element with constant frequency differences and some observations 

 have been made by Purvis^ with the Zeeman effect on the wavelengths 

 in its spectrum which should prove useful in identifying series. 



The studies made by De Watteville^ on the flame spectrum of 

 lead and of its salts should also aid in picking out those wavelengths, 

 at least, which constitute fundamental series. With a number of the 

 elements it has been found that the quantum relation Ve = hn 

 enables one when the resonance and ionisation potentials are known 

 to pick out the first and the last members of the principal singlet 

 series n= (1 -SjS) — (m,P). As regards lead, Mohler, Foote, and 

 Stimson^ have recently shown by direct experiments on the ionisation 

 of lead vapour that the resonance and ionisation potentials are 

 respectively 1-26 volts and 7-93 volts. 



Assuming the use of the quantum relation indicated above to be 

 valid for the case of lead it would follow that the first member of the 

 series n = (1 •5,S) — (m,P) should be one or other of the strong Unes 

 A = 10291 A°U or A =10500 A°U, which are both present in the lead 

 spectrum. It would also follow that the last member of this series 



» Purvis, Proc. Camb. Phil. Soc, Vol. 14, p. 216, 1907. 



» De Watteville Phil. Trans. A375, p. 139, 1904. 



3 Mohler, Foote and Stimson, Phys. Rev. 14, p. 534. 1919. 



;Sec. Ill, Sig. 2 



