Section III, 1919 [7] Trans. R.S.C. 



On the Absorption Spectra of Thallium, Aluminium, Lead and Tin, 



and Arsenic 



By Professor J. C. McLennan, F.R.S., Mr. J. F. T. Young, M.A". 

 and Mr. H. J. C. Ireton, M.A. 



(Read May Meeting, 1919.) 



I. Introduction 



Recent work by Foote, Rognley and Mohler^ on the resonance and 

 ionization potentials of thallium vapour by electrical methods gave 

 the values 1-3 and 7-3 volts respectively for these magnitudes. In 

 previous work by McLennan and others^ it has been found that the 

 resonance potential for certain elements is related by the quantum 

 relation ve = hv to the lowest frequency of the series v = (l-5 S) 



— (m, P2) and that the ionization potential is similarly given by 

 applying the same relation to the limiting frequency of the series 

 v=(l-5 S) — (m, P). By analogy it was thought that the values of 

 the potentials obtained for thallium could be used to determine these 

 two series. The resonance voltage given above corresponds to radi- 

 ation of wavelength 11, 513 A.U. which is a well known line in the 

 infra-red while the ionization voltage gives the wavelength 1700 A.U. 

 Reference to a recent paper by McLennan, Ainslie and Fuller^ shows 

 that no line had as yet been found with the frequency of the latter 

 in the spectrum of thallium. 



In a previous paper by two of the authors* the method of arc 

 reversals was successfully applied in determining the series v = (1 -5, S) 



— (m.P) for calcium strontium and barium. It was thought, therefore, 

 that possibly further knowledge of the series of thallium and other 

 metals might be obtained by an application of this method. 



II. Experimental Arrangements 



A small Hilger quartz spectrograph Type A and Schumann 

 plates prepared by the Adam Hilger Co., were used in taking the photo- 

 graphs of the spectra. The best results were obtained by focussing the 

 light from the source on the slit with a cylindrical quartz lens. The 

 arc arrangements were the same as that described in a previous paper.^ 



1 Foote, Rognley & Mohler, Phys. Rev. Vol. XIII, No. 1, Jan., 1919, p. 59. 

 ^Guthrie Lecture by Prof. McLennan, Proc. Phys. Soc. London, Vol. I, pt. I, 

 Dec. 15, 1918. Tate, Phys. Rev. Vol. X, No. 1, p. 81, 1917. 

 3 Proc. Roy. Soc. Sec. A. Vol. 95, Mar. 15, 1919, p. 316. 



* McLennan & Young, Proc. Roy. Soc. A. Vol. 95, 1919. 



* McLennan & Young, loc. cit. 



