10 THE ROYAL SOCIETY OF CANADA 



should have approximately the frequency 64400 and the wave 

 length A =1558 A°U. The experiments of Dearie,^ however, have 

 shown that with mercury vapour a resonance potential probably 

 exists which corresponds approximately to n= (2 -5,5) — (2,P), the 

 frequency of the first member of the series n = (2-5,S) — (m,P). It 

 'may very well be then that the resonance potential found by Mohler, 

 Foote, and Stimson for lead viz.: 1-26 volts, corresponds to the 

 spectral frequency n = (2-5,S) — (2,P) for this element. 



Some additional light has been shed on the problem of identifying 

 spectral series for lead through some observations made by McLennan, 

 Young, and Ireton on the spectrum obtained from lead when vapour- 

 ised in the carbon arc. In these experiments a number of reversals 

 were obtained, and from the character and positions of these the 

 above named investigators were led to put forward tentatively 

 certain groupings of the wavelengths as possible ones for series. 



The following paper contains the results of an attempt to follow 

 up these suggestions. 



II. Absorption Experiments 



Some experiments were made to determine if possible the absorp- 

 tion spectrum of non-luminous lead vapour. Considerable difficulty 

 was experienced at first in obtaining any results on account of the 

 fact that, although lead melts at a very low temperature, it does not 

 vapourise to any great extent, even in vacuo, until a temperature is 

 reached approximately the same as that at which quartz softens. An 

 absorption spectrum was, however, finally obtained by using a quartz 

 tube of special design and heating it in an electric furnace, provided 

 with a coil of stout nichrome wire. 



The tube was made of opaque fused quartz about 5 inches in 

 length and one inch in diameter, and into its ends there was sealed 

 two plates of plane parallel clear fused quartz. This tube stood up to 

 the high temperatures required and these were obtained by forcing 

 large currents through the stout nichrome wire. For the purpose of 

 the experiments some lead was obtained from the Bureau of Standards 

 at Washington which was considered to be of very high purity. About 

 50 grams of this lead was placed in the quartz tube and the latter was 

 then highly exhausted and sealed up. 



The wavelengths at which absorption was found are given in 

 Table I, and with them are given the wavelengths which showed 

 reversal in the experiments of McLennan, Young, and Ireton. The 

 photographic plates used were either of the Ilford panchromatic type 

 or those of the Schumann type prepared by The Adam Hilger Co. 



